Cavity Receivers for Parabolic Solar Troughs

ABSTRACT

A tubular heat-absorbing element partly enclosed in an insulating layer or jacket, has absorbing surface that is accessible to solar radiation. The thermal insulation is designed to provide entry to solar radiation by way of a cavity. The absorbing surface can be substantially planar.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of co-pending U.S.provisional patent application Ser. No. 61/583,585, filed Jan. 5, 2012,which application is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

This invention was made with government support under DE-EE0005803awarded by the DOE. The government has certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to concentrating solar power in general andparticularly to a system that employs reflective troughs to focus solarradiation upon a linear receiver.

BACKGROUND OF THE INVENTION

Solar parabolic troughs, which focus sunlight on tubes carrying a fluidthat conveys heat for steam generation (e.g. to a steam-driven electricgenerator, or for industrial process heat) or to a body of material forenergy storage, are a proven, reliable, and relatively low-costtechnology for collecting energy. The tubes upon which the light isfocused in such systems are typically termed “receiver tubes,”“receivers,” or “heat collection elements.” At a typical electricgeneration plant employing solar parabolic troughs, many receivers(e.g., thousands) are arrayed with reflective troughs in parallel rowsto form a “field” that can collect sufficient energy for a generatingsystem of economical size. At present, receivers represent approximately12% of the capital cost of a concentrating solar power installationemploying solar parabolic troughs.

In a typical receiver constructed according to the prior art, a centralliquid-carrying tube with an outer optical absorption coating issurrounded by a vacuum held within a transparent concentric envelope.Light focused on the receiver by a mirror (also known as a “collector”)passes through the transparent concentric envelope and through thevacuum and impinges on the central liquid-carrying tube. The coating onthe central tube absorbs most (preferably all, although this cannot berealized in practice) of the energy incident upon it and is thus heated.This heat is transmitted by conduction through the wall of the centraltube and thence to the tube's liquid contents. The heated liquid ispumped through the receiver and, in general, through additionalreceivers, being thus raised to a high temperature (e.g., 400° C.)before being pumped to a boiler, or energy storage device (e.g.,reservoir of hot fluid). The function of the vacuum between the inner,fluid-carrying tube and the outer, transparent envelope is to preventloss of heat from the receiver by convection and conduction to the outerenvelope and thence, by radiation and conduction, to the environment.

A number of problems in the use of standard vacuum-containing receivertubes have been observed. These include, but are not limited to, thefollowing: (a) The absorption coatings on the inner, fluid-carrying tubeare expensive to manufacture. (b) Degradation of a receiver's vacuumentails increased thermal losses from the receiver and, if severeenough, requires replacement of the receiver. In practice, vacuumdegradation causes failure of 1-5% of receiver tubes per year. (c) Thetubular outer glass envelope of a conventional receiver must be thickenough to withstand the stresses imposed by containing a vacuum as wellas by wind and its own weight. This strength requirement increases thecost of the envelope. (d) The absorptive coating upon the inner,fluid-containing tube of a receiver not only absorbs radiant energy butemits it, particularly in the infrared part of the spectrum. Emissionlosses increase with coating temperature T approximately as the fourthpower of T (i.e., as T⁴). Energy thus emitted is for the most part lostto the environment, diminishing the receiver's efficiency. Moreover, theabsorptive coating may be destroyed by sufficiently high T.Prohibitively large T⁴ radiation losses, coupled with high-temperatureinstability of the absorber coating, today prevent practical operationof solar parabolic-trough generating plants at elevated temperatures(>500° C.). Yet, for fundamental thermodynamic reasons it is moreefficient to operate any thermal generating plant at higher peak T.

There is thus a need for receivers for parabolic-trough solar power thatare less costly to acquire and maintain than are receivers constructedaccording to the prior art and that allow operation at highertemperature.

SUMMARY OF THE INVENTION

In particular, the invention addresses the following issues and providesadvantages over the prior art. Receivers are provided that, relative tostate-of-the-art receivers, do not suffer from vacuum degradation, whoseabsorptive coatings and other components are less costly and simpler tomanufacture, that operate durably with undiminished or improved overallenergetic efficiency at today's typical operating temperatures (e.g.,350° C.) or at higher temperatures (e.g., at 600° C. or above), and thatcan be used either in the construction of new power plants or asdrop-replacements for defunct conventional receivers in existing powerplants. Such improved receivers are expected to lower the levelized costof energy for concentrating solar power produced using solar parabolictroughs.

Moreover, for most thermal energy storage systems, the operationaltemperature range has an effect on the cost of storage: that is,increased temperature tends to decrease cost. The increased operatingtemperature enabled by embodiments of the invention will reduce theamount of thermal storage media required for thermal energy storagecompared to the current approaches known in the art, which, for linear,trough-style collection systems, typically operate in the range of 300°C.-400° C. For example, for a system employing salt (or other thermalstorage medium storing thermal energy in sensible heat capacity, such asgravel, ceramics, oils, and other solids and fluids) as the thermalstorage media, the mass of salt required to store a given quantity ofenergy is inversely proportional to the temperature differential in thestorage system. Thus, a trough system operating from 300° C.-600° C.requires approximately three times less storage salt for a givenquantity of energy than does a trough system operating from 300° C.-400°C. This reduction in storage-material mass and the associated reductionin costs make it possible to economically add higher thermal-energystorage capacities.

As used hereinafter, a “tube” or object having a “tubular” form is anyelongated, two-ended, hollow body whose cross-sectional form is a simpleclosed figure (e.g., circle, rectangle, rectangle with rounded corners);a tube may be either closed or open at its ends. The invention pertainsto a tubular receiver or heat-absorbing element for use in concentratingsolar power systems. Light is focused along the length of the receiverby a trough-shaped collector having a reflective surface that istypically parabolic in cross-section.

According to one aspect, the invention features a central tube or pipe(herein also termed “the radiation-absorbing element”) through which afluid heat-transfer medium flows. A portion of the energy focused uponthe receiver by the collector is ultimately absorbed by the fluid medium(e.g., the fluid medium is heated and/or undergoes a phase change). Thefluid is then circulated through the radiation-absorbing element andcirculated through piping in order to transport the heat energy to aboiler, to a storage unit, or to another destination.

The invention also features a solar absorber coating on part or all ofthe exterior surface of the radiation-absorbing element. The portion ofthe radiation-absorbing element's surface that is coated with theabsorbent coating is herein termed the “absorbing surface.” The solarabsorber coating is designed to absorb a large portion of the light thatimpinges upon it, converting this energy to the form of heat; isdesigned to be stable at temperatures up to and in excess of 400° C.;and is designed to have both high optical absorptance and low thermalemissivity. That is, the solar absorber coating effectively absorbslight, especially in the visible part of the spectrum, but tends not tore-radiate the energy thus absorbed as infrared light. Thus, energycollected by the radiation-absorbing element tends to be retained ratherthan dissipated to the environment in the form of infrared radiation.

The invention also features a substantially opaque, thermally insulatingjacket around at least a portion of the radiation-absorbing element.

The invention also features a second tube (herein termed the “shell”)that surrounds the radiation-absorbing element and thermally insulatingjacket. The shell admits light through a non-opaque strip or segment(herein also termed the “aperture”) that runs lengthwise along theshell.

In various embodiments, the aperture may be covered partly or wholly byone or more strips of a solid, transparent material (e.g., glass) or mayconsist partly or wholly of an unobstructed opening. The heat-absorbingelement is located within the shell, aligned with the shell, andseparated from the shell by an intervening space at most points. Thespace between the pipe and the shell is herein also termed the “cavity.”The absorbing surface of the heat-absorbing element is exposed to lightthat enters through the aperture. Light that has entered through theaperture may impinge directly on the absorbing surface, or may undergoone or more reflections or be absorbed and re-emitted one or more timesbefore impinging on the absorbing surface. The portion of the cavitythrough which light can pass after entering the aperture or beingreflected or re-emitted within the cavity is herein termed the “opticalcavity”; the absorbing surface is exposed to (i.e., forms one surface orwall of) the optical cavity. The portion of the cavity between theheat-absorbing element and the shell that is not the optical cavity isfilled with the thermally insulating jacket and is herein termed the“insulation cavity.” Portions of the cavity not occupied by some solidmaterial are occupied by a gas (e.g., ordinary air) at or near ambientatmospheric pressure.

In one embodiment, the optical cavity and the insulation cavity areseparated by two barriers that are herein termed the “sidewalls.” Thesidewalls bound the sides of the optical cavity and may consistessentially of strips of a relatively thin material. The sidewallsprevent or impede the mixing of gas in the insulation cavity with gas inthe optical cavity. The aperture, the absorbing surface, and thesidewalls are positioned and sized so that when the receiver isapproximately aligned with the focus of a trough-shaped mirror ofspecific dimensions, light reflected from any portion of the mirrorenters the aperture along a path that leads directly to the absorbingsurface of the collector. Light focused from the mirror does not impingesubstantially upon the sidewalls. The sidewalls may be opaquelyabsorbent, opaquely reflective, or transparent.

In another embodiment, the optical cavity and the insulation cavity areseparated by sidewalls that are parabolic in cross-section. Eachsidewall is reflective on the side facing the optical cavity. Theaperture, absorbing surface, and parabolic sidewalls are sized andshaped so that when the receiver is placed approximately at the focus ofa trough-shaped mirror of specific dimensions, light reflected fromevery portion of the mirror either (a) enters the aperture along a paththat leads directly to the absorbing surface or (b) after entering theaperture is reflected from a parabolic sidewall and then impinges on theabsorbing surface.

In yet another embodiment, sidewalls bound the optical cavity on twosides and the opening is covered wholly or partly by a linear strip oftransparent material (e.g., glass) having a cross-section that causesthe strip to act as a lens. The lens, aperture, absorbing surface, andsidewalls are positioned and sized so that when the receiver isapproximately aligned with the focus of a trough-shaped mirror ofspecific dimensions, light reflected from every portion of the mirrorimpinges on the lens and is refracted thereby into the optical cavity.Light reflected by any portion of the mirror and passing through thelens does not impinge substantially upon the sidewalls. The sidewallsmay be opaquely absorbent, opaquely reflective, or transparent.

In a further embodiment, the absorbing surface of the heat-absorbingelement is planar or approximately planar.

In still another embodiment, sidewalls bound the optical cavity on twosides and the receiver features a concentric outer tube (herein alsotermed “the cover”) that encloses the shell. The cover includes atransparent strip or segment (herein also termed “the window”) that runsalong its length. The window may consist substantially of a transparentsolid material or an opening. The cover tube also includes an opaquestrip or segment (herein also termed “the cap”) that runs along itslength. The cap may feature one or more of a reflective inward-facingsurface and an insulating layer. Both the window and the cap are wideenough and long enough to at least cover the aperture. The cover may berotated around its long axis so as to bring either the cover window orthe cap into alignment with the aperture. The function of the window isto admit light into the optical cavity; the function of the cap is toprevent or reduce radiative and/or convective losses of energy from theinterior of the receiver. To admit light into the receiver, the cover isrotated so that the cover window is over the aperture. To conserve heatwithin the receiver, the cover is rotated so that the cap is over theaperture. The cover window (if solid) and/or the cap may make apartially or wholly airtight seal with the shell of the receiver.

In a further embodiment, the outer surface of the receiver (e.g., invarious embodiments, the shell; e.g., in various other embodiments, thecover) is shaped aerodynamically, i.e., in a manner that minimizes theaverage force exerted upon the receiver by winds.

According to one aspect, the invention features a linear solar receiverfor use in a concentrating solar power system. The linear solar receivercomprises a solar radiation absorbing element having an outer surfaceconfigured to circumscribe an interior volume, the interior volumedesigned to contain a heat transfer medium, the solar radiationabsorbing element designed to absorb an incident flux of solar radiationand transfer an absorbed flux of energy to the heat transfer medium, theheat transfer medium designed to receive and transport at least aportion of the absorbed flux of energy, the heat transfer medium whentransporting at least a portion of the absorbed flux of energy beingprimarily in a fluid phase; a solar selective absorber located on afirst portion of the outer surface of the solar radiation absorbingelement, the solar selective absorber having a thermal emittance valueand an optical absorptance value, the optical absorptance value beingdifferent from the thermal emittance value, being exposed to ambientatmospheric pressure; an substantially opaque thermally insulatingjacket, the substantially opaque thermally insulating jacket in contactwith a second portion of the outer surface of the solar radiationabsorbing element; and a solar radiation admitting region having aninterior surface, at least a portion of the solar radiation admittingregion being surrounded by at least a portion of the substantiallyopaque thermally insulating jacket, the solar radiation admitting regiondesigned to allow transmission of at least a portion of the incidentflux of solar radiation to be incident on the solar selective absorber,the solar radiation admitting region being symmetric with respect to aplane parallel to a length dimension of the solar radiation absorbingelement, the plane oriented in a perpendicular direction to the outersurface of the solar radiation absorbing element.

According to another aspect, the invention relates to a linear solarreceiver for use in a concentrating solar power system. The linear solarreceiver comprises a solar radiation absorbing element having an outersurface configured to circumscribe an interior volume, the interiorvolume designed to contain a heat transfer medium, the solar radiationabsorbing element designed to absorb an incident flux of solar radiationand transfer an absorbed flux of energy to the heat transfer medium, theheat transfer medium designed to receive and transport at least aportion of the absorbed flux of energy, the heat transfer medium whentransporting at least a portion of the absorbed flux of energy beingprimarily in a fluid phase, the solar radiation; a solar selectiveabsorber located on a first portion of the outer surface of the solarradiation absorbing element, the solar selective absorber having athermal emittance value and an optical absorptance value, the opticalabsorptance value being different from the thermal emittance value, thesolar selective absorber being exposed to ambient atmospheric pressure;an substantially opaque thermally insulating jacket, the substantiallyopaque thermally insulating jacket in contact with a second portion ofthe outer surface of the solar radiation absorbing element; and a solarradiation admitting region having an interior surface, at least aportion of the solar radiation admitting region being surrounded by atleast a portion of the substantially opaque thermally insulating jacket,the solar radiation admitting region designed to allow transmission ofat least a portion of the incident flux of solar radiation to beincident on the solar selective absorber, wherein the receiver isdesigned to be located symmetrically between outer edges of a solarmirror collector system such that at least a portion of the receivershades at least a portion of the solar mirror collector from incidentsolar radiation.

According to another aspect, the invention relates to a system forgenerating energy from solar radiation as part of a solar power system.The system comprises a plurality of linear receivers, each of theplurality of linear receivers including at least a solar radiationabsorbing element designed to absorb an incident flux of solar radiationand transfer an absorbed flux of energy to a heat transfer medium, theheat transfer medium designed to receive and transport at least aportion of the absorbed flux of energy, at least a portion of theradiation absorbing element being covered with a solar selectiveabsorber, the solar selective absorber having a thermal emittance valueand an optical absorptance value, the optical absorptance value beingdifferent from the thermal emittance value; a parabolic trough mirrorcollector for concentrating solar radiation onto the plurality of linearreceivers; a control system for directing the parabolic trough mirror atthe sun, wherein the heat transfer medium circulating in a firstreceiver in the plurality of linear receivers is heated by solarradiation from a first elevated temperature T1 to a second elevatedtemperature T2 over a first distance corresponding to a length of thefirst receiver and the heat transfer medium circulating in a secondreceiver in the plurality of linear receivers is heated by solarradiation from a third elevated temperature T3 to a fourth elevatedtemperature T4 over a second distance corresponding to a length of thesecond receiver, where T4>T3≧T2>T1, the first receiver and the secondreceiver having structures designed for operation in differenttemperature ranges.

According to another aspect, the invention relates to a concentratingsolar power system. The system comprises a plurality of linear solarreceivers connected in series, the plurality of linear solar receiversconnected in series arranged such that a heat transfer medium flowingtherethrough exhibits an increase in temperature as it passes from afirst end to a second end of the plurality of linear receivers connectedin series, at least one of the plurality of linear receivers comprising:a solar radiation absorbing element having an outer surface configuredto circumscribe an interior volume, the interior volume designed tocontain a heat transfer medium, the solar radiation absorbing elementdesigned to absorb an incident flux of solar radiation and transfer anabsorbed flux of energy to the heat transfer medium, the heat transfermedium designed to receive and transport at least a portion of theabsorbed flux of energy, the heat transfer medium when transporting atleast a portion of the absorbed flux of energy being primarily in afluid phase; a solar selective absorber located on a first portion ofthe outer surface of the solar radiation absorbing element, the solarselective absorber having a thermal emittance value and an opticalabsorptance value, the optical absorptance value being different fromthe thermal emittance value, the solar selective absorber being exposedto ambient atmospheric pressure; an substantially opaque thermallyinsulating jacket, the substantially opaque thermally insulating jacketin contact with a second portion of the outer surface of the solarradiation absorbing element; and a solar radiation admitting regionhaving an interior surface, at least a portion of the solar radiationadmitting region being surrounded by at least a portion of thesubstantially opaque thermally insulating jacket, the solar radiationadmitting region designed to allow transmission of at least a portion ofthe incident flux of solar radiation to be incident on the solarselective absorber, the solar radiation admitting region being symmetricwith respect to a plane parallel to a length dimension of the solarradiation absorbing element, the plane oriented in a perpendiculardirection to the outer surface of the solar radiation absorbing element;a parabolic trough reflector in optical registry with the at least oneof the plurality of linear receivers such that when incident solarradiation falls on the parabolic trough reflector, the incident solarradiation is directed to the solar radiation admitting region; anddevice configured to extract thermal energy from the heat transfermedium that exits the second end of the plurality of linear receiversconnected in series, thereby cooling the heat transfer medium, andconfigured to return cooled heat transfer medium to the first end of theplurality of linear receivers connected in series.

According to another aspect, the invention relates to a linear solarreceiver for use in a concentrating solar power system. The receivercomprises a solar radiation absorbing element having an outer surfaceconfigured to circumscribe an interior volume, the interior volumedesigned to contain a heat transfer medium, the solar radiationabsorbing element designed to absorb an incident flux of solar radiationand transfer an absorbed flux of energy to the heat transfer medium, theheat transfer medium designed to receive and transport at least aportion of the absorbed flux of energy, the heat transfer medium whentransporting at least a portion of the absorbed flux of energy beingprimarily in a fluid phase; a solar selective absorber located on afirst substantially planar portion of the outer surface of the solarradiation absorbing element, the solar selective absorber having athermal emittance value and an optical absorptance value, the opticalabsorptance value being different from the thermal emittance value, thesolar selective absorber being exposed to ambient atmospheric pressure;an substantially opaque thermally insulating jacket, the substantiallyopaque thermally insulating jacket in contact with a second portion ofthe outer surface of the solar radiation absorbing element, the secondportion of the outer surface of the solar radiation absorbing elementcomprising at least 50% of an area of the outer surface of the solarradiation absorbing element determined on a per unit length basis; asolar radiation admitting region having an interior surface defined inan external region, at least a portion of the external region in contactwith at least a portion of the substantially opaque thermally insulatingjacket, the solar radiation admitting region designed to allowtransmission of at least a portion of the incident flux of solarradiation to be incident on the solar selective absorber; and aparabolic mirror collector having a rim angle of less than 75 degrees,the parabolic mirror collector configured to reflect solar radiation onto the solar radiation admitting region.

According to another aspect, the invention relates to a linear solarreceiver for use in a concentrating solar power system. The receivercomprises a solar radiation absorbing element having an outer surfaceconfigured to circumscribe an interior volume, the interior volumedesigned to contain a heat transfer medium, the solar radiationabsorbing element designed to absorb an incident flux of solar radiationand transfer an absorbed flux of energy to the heat transfer medium, theheat transfer medium designed to receive and transport at least aportion of the absorbed flux of energy, the heat transfer medium whentransporting at least a portion of the absorbed flux of energy beingprimarily in a fluid phase; a solar selective absorber located on thesolar radiation absorbing element, the solar selective absorber having athermal emittance value and an optical absorptance value, the opticalabsorptance value being different from the thermal emittance value; andwherein the receiver has a thermal efficiency defined as one minus aheat loss divided by the absorbed flux of energy, the thermal efficiencyequal to or greater than at least one of 94 percent at 450 degreesCelsius and 92 percent at 500 degrees Celsius.

In one embodiment of the apparatus as previously described, the firstportion of the outer surface of the solar radiation absorbing element issubstantially planar.

In another embodiment of the apparatus as previously described, thefirst portion of the outer surface of the solar radiation absorbingelement comprises a fraction in the range of 0.50 to 0.20 of an area ofthe outer surface of the solar radiation absorbing element determined ona per unit length basis.

In another embodiment of the apparatus as previously described, theapparatus further comprises a symmetric parabolic trough collectormirror having a rim angle of less than 75 degrees.

In another embodiment of the apparatus as previously described, theinterior surface of the solar radiation admitting region issubstantially parabolic in cross section as viewed parallel to thelength dimension of the solar radiation absorbing element.

In another embodiment of the apparatus as previously described, theinterior surface of the solar radiation admitting region is a reflectivesurface.

In another embodiment of the apparatus as previously described, the heattransfer medium is selected from the group consisting of a heat transfersalt, a low-melting-point inorganic nitrate salt fluid, a hybrid organicsiloxane-based fluid, a molecular silicone-based fluid, an oil andsteam.

In another embodiment of the apparatus as previously described, athermal efficiency of the linear solar receiver as a function oftemperature is at least as high as given in any of the four rightmostcolumns of Table 1.

In another embodiment of the apparatus as previously described, theapparatus appears in combination with an energy collection systemconfigured to operate an energy recovery machine that relies upon theCarnot cycle for recovery of energy from the heat transfer fluid.

In another embodiment of the apparatus as previously described, theapparatus appears in combination with a machine that generates steam.

In another embodiment of the apparatus as previously described, theapparatus appears in combination with a machine that generateselectricity.

In another embodiment of the apparatus as previously described, theapparatus appears in combination with a thermal energy storage device.

In another embodiment of the apparatus as previously described, theapparatus appears in combination with a controller that controls therate of generation of energy.

In another embodiment of the apparatus as previously described, theapparatus has a thermal efficiency and an optical efficiency such thatincreasing the thermal efficiency by increasing a thickness of thesubstantially opaque thermally insulating jacket decreases the opticalefficiency due to increased shading of the collector by the receiver.

In another embodiment of the apparatus as previously described, thesolar radiation absorbing element, substantially opaque thermallyinsulating jacket, and the solar radiation admitting region aresymmetric with respect to a bisecting plane that is parallel to the axisof the linear solar receiver.

In another embodiment of the apparatus as previously described, theapparatus further comprises a glass cover enclosing the solar radiationadmitting region.

In another embodiment of the apparatus as previously described, an inertgas is introduced into the radiation admitting region.

In another embodiment of the apparatus as previously described, theapparatus appears in combination with a plurality linear solarreceivers, the linear solar receivers each including at least solarradiation absorbing elements, adjacent solar radiation absorbingelements forming a nearly continuous absorbing surface.

In another embodiment of the apparatus as previously described, theapparatus appears in combination with a plurality linear solarreceivers, a first one of the plurality of receivers operating at afirst temperature and a second one of the plurality of receiversoperating at a second temperature, the first receiver and the second ofthe plurality of receivers having different designs, the first and thesecond temperatures being different.

In another embodiment of the apparatus as previously described, theapparatus further comprises a symmetric parabolic trough collectormirror structure, the symmetric parabolic trough collector mirrorstructure being held in a substantially rigid form with cablesuspension.

In another embodiment of the apparatus as previously described, thesolar selective absorber is a plasmonic nanochain cermet structure.

According to another aspect, the invention relates to a method ofgenerating energy from solar radiation. The method comprises the stepsof concentrating a flux of solar radiation incident on a parabolictrough collector mirror onto a linear receiver, the linear receiverbeing symmetric with respect to a plane bisecting the linear receiverand parallel to a linear axis of the linear receiver, the linearreceiver including at least a heat transfer conduit; absorbing a portionof the flux of solar radiation with a solar selective absorber disposedon at least a portion of the heat transfer conduit, the solar selectiveabsorber having a thermal emittance value and an optical absorptancevalue, the optical absorptance value being different from the thermalemittance value, the portion of the flux of solar radiation constitutingabsorbed solar radiation; heating a heat transfer medium circulatingwith the heat transfer conduit to a temperature exceeding 350 degreesCelsius by transferring a first portion of the absorbed solar radiationto the heat transfer medium, the first portion of the absorbed solarradiation constituting transferred solar radiation; controlling theparabolic trough collector mirror with a control system to maintaindirectional focus at the sun, wherein the receiver has a thermalefficiency defined as one minus a heat loss divided by the absorbed fluxof energy, the thermal efficiency equal to or greater than at least oneof 94 percent at 450 degrees Celsius and 92 percent at 500 degreesCelsius.

According to another aspect, the invention relates to a method ofgenerating energy from solar radiation. The method comprises the stepsof concentrating a flux of solar radiation incident on a parabolictrough collector mirror onto a linear receiver; controlling theparabolic trough collector mirror with a control system to maintaindirectional focus at the sun; and maintaining the parabolic troughcollector mirror in a rigid shape with a cable suspension system, thecable suspension system attached to an upper suspension element abovethe parabolic trough collector mirror and attached to a lower suspensionelement below the parabolic trough collector mirror, the uppersuspension element and the lower suspension element attached to aplurality of support elements, the plurality of support elementsattached to a frame of the trough collector mirror.

In another embodiment of the method as previously described, a weight ofthe cable suspension system is less than a support structure of atraditional parabolic trough mirror collector.

In another embodiment of the method as previously described, at least aportion of the linear receiver functions as the upper suspensionelement.

In another embodiment of the method as previously described, the linearreceiver includes at least an absorber conduit recessed withinsubstantially opaque insulation.

In another embodiment of the method as previously described, the thermalefficiency is greater than or equal to at least one of 89 percent at 550degrees Celsius, 85 percent at 600 degrees Celsius, and 80 percent at650 degrees Celsius.

In another embodiment of the method as previously described, the atleast a portion of the heat transfer conduit having the solar selectiveabsorber disposed is substantially planar.

In another embodiment of the method as previously described, a portionof the heat transfer conduit is covered by substantially opaque thermalinsulation.

In another embodiment of the method as previously described, at least aportion of the substantially opaque thermal insulation has a thermalconductivity of less than 40 milliWatts per meter per degree Kelvin.

In another embodiment of the method as previously described, thesubstantially opaque thermal insulation is pyrogenic silica.

In another embodiment of the method as previously described, the solarselective absorber is a plasmonic nanochain cermet structure.

In another embodiment of the method as previously described, theplasmonic nanochain cermet structure is a Ni nanochain-Al₂O₃ cermet.

In another embodiment of the method as previously described, theparabolic trough collector mirror has a rim angle less than 75 degrees.

These and other objects, along with the advantages and features of thepresent invention herein disclosed, will become apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1 is a schematic diagram of a concentrating solar-power electricalgeneration facility.

FIG. 2 illustrates exemplary embodiments of a system supporting aparabolic trough mirror and receiver tube.

FIG. 3A and FIG. 3B illustrates aspects of the prior art forvacuum-containing receiver tubes.

FIG. 4 is a cross sectional view of a receiver according to principlesof the invention.

FIG. 5A, PLOT 5A is a graph that shows the receiver thermal efficiencyas a function of temperature for a prior art receiver and a receiverconstructed according to the principles of the present invention.

FIG. 5A, PLOT 5B is a graph that shows the receiver total efficiency asa function of temperature for a prior art receiver and a receiverconstructed according to the principles of the present invention.

FIG. 5B is a cross-sectional plot of simulated contours of constanttemperature in a state-of-the-art receiver.

FIG. 5C is a graph of heat losses for a receiver built according to theprior art under three different conditions of operation.

FIG. 6 is a graph comparing experimental and extrapolated data tosimulated data, showing that the simulation produces realistic results.

FIG. 7 is a schematic diagram of an illustrative embodiment of anadvanced cavity receiver according to the invention.

FIG. 8 is a graph of lines of constant temperature in a simulation ofthree illustrative embodiments of the invention.

FIG. 9 is a graph of the heat loss per unit length of a receiveremploying the prior art as compared to that of illustrative receiversembodying aspects of the invention.

FIG. 10 illustrates terms used herein to describe the geometry of areceiver mounted above a collector.

FIG. 11 is a schematic diagram of an illustrative embodiment of anadvanced cavity receiver according to the invention, including alenticular aperture cover.

FIG. 12 is a schematic diagram of an illustrative embodiment of anadvanced cavity receiver according to the invention, including adifferent lenticular aperture cover.

FIG. 13A through FIG. 13F show ray tracings for light focused upon aconcentric-tube receiver geometry.

FIG. 14A through FIG. 14C show ray tracings for light focused upon aconcentric-tube receiver geometry, incorporating collector focusingerrors.

FIG. 15A and FIG. 15B are graphs of the optical efficiency of a systemas a function of collector focal length and the diameter of thereceiver's absorber absorber for specified collector errors.

FIG. 16 is a graph of the relationship between system concentrationratio (focal spot size) and collector rim angle for a receiver.

FIG. 17A and FIG. 17B graph the optical official of a receiver having aplanar absorbing surface for a range of absorbing-surface widths,collector focal lengths and glass-covered or uncovered apertures.

FIG. 18 illustrates the geometry of a compound parabolic concentrator.

FIG. 19A1 and FIG. 19A2 graph the optical efficiencies of receiverhaving a planar absorbing surface, 100% or 95% reflective compoundparabolic concentrators, with an uncovered aperture.

FIG. 19B1 and FIG. 19B2 graph the optical efficiencies of receiverhaving a planar absorbing surface, 100% or 95% reflective compoundparabolic concentrators, and a glass-covered aperture.

FIG. 20A and FIG. 20B display illustrative graphs of energy gained andlost by a receiver as a function of absorbing-surface width.

FIG. 21A is a schematic diagram of an illustrative receiver.

FIG. 21B is a graph of heat loss as a function of temperature for thisreceiver and others.

FIG. 22A1, FIG. 22A2, FIG. 22A3 and FIG. 22A4 graph the circulation ofheat within, a receiver partially covered by insulation and a receivernot covered by insulation.

FIG. 22B1 and FIG. 22B2 show categories of heat loss from a receiverpartially covered by insulation.

FIG. 23A graphs the circulation of heat within a receiver havinginternal dividers.

FIG. 23B graphs the convection heat loss from, a receiver havinginternal dividers.

FIG. 24A1 is a schematic diagram of a receiver having internalinsulation.

FIG. 24A2 graphs the circulation of heat within a receiver havinginternal insulation.

FIG. 24B1 shows categories of heat loss for a receiver with internalinsulation.

FIG. 24B2 graphs heat loss as a function of temperature for a receiverwith internal insulation and others.

FIG. 25A, FIG. 25B, and FIG. 25C graph the circulation of heat withinthe optical cavity of, and heat loss from, a cavity receiver at variousangles of tilt.

FIG. 26A is a graph of lines of constant temperature for a cavityreceiver both without an external cover over its cavity.

FIG. 26B is a graph of lines of constant temperature for a cavityreceiver with an external cover over its cavity.

FIG. 27 is a schematic diagram of an illustrative cavity receiver withprovisions for moving an external cover into place over its cavity andremoving said cover at will.

FIG. 28A, FIG. 28B, FIG. 28C and FIG. 28D are graphs of contours ofconstant velocities from an illustrative receiver when blown upon bywind at various angles.

FIG. 29 is a table of heat losses from various receivers when blown uponby wind at various angles.

FIG. 30A and FIG. 30B are schematic diagrams of illustrative cavityreceivers having partially covered cavities.

FIG. 31 is a schematic diagram of a solar field in which a graded seriesof cavity receivers is included in each row of the field.

FIG. 32 is a schematic diagram of a solar field in which a graded seriesof cavity receivers is included in each row of the field, in addition toreceivers employing prior art.

FIG. 33 is a schematic diagram of a method for supporting a receiverabove a collector.

FIG. 34A is a schematic diagram of an optical absorbing layer applied toa substrate.

FIG. 34B is a schematic diagram of an optical absorbing layer applied toa substrate.

FIG. 35 is a graph of extinction, absorption, and scattering for opticalabsorbing layers that include nickel nanochains of various lengths.

FIG. 36 is a pair of photomicrographs showing exemplary nickel nanochaincompounds.

FIG. 37 is a graph of the reflectance of a plasmonic nanochain cermetstructure.

FIG. 38A and FIG. 38B are, respectively, a schematic drawing of themolecular structure of hydrogen silsesquioxane and a graph of X-raydiffraction intensity of hydrogen silsesquioxane as a function of time.

FIG. 39 is a graph of estimated component costs of some embodiments ofthe invention compared to costs for the state of the art.

DETAILED DESCRIPTION

FIG. 1 depicts an illustrative system 100 for the collection of solarenergy in the form of heat and the conversion of the collected heat toelectricity. Subsequent figures will clarify the application ofembodiments of the invention to such a system and similar systems whichuse the collected heat in other manners, such as to generate steam forindustrial processes. The system 100 depicted in FIG. 1 comprises asolar field 102 for the collection of solar energy. The solar field 102comprises a plurality N rows (e.g., row 104, row 106), depicted in FIG.1 as viewed from above, where each row comprises a plurality M receivers(e.g., receivers 108 a, 108 b; receivers 110 a, 110 b, etc.) upon whichsolar radiation is focused by parabolic collectors (not depicted). Theplurality of the N rows is indicated in FIG. 1 by horizontal ellipses(e.g., ellipsis 112); the plurality of M receivers in each of the N rowsis indicated in FIG. 1 by vertical ellipses (e.g., ellipsis 114). Thenumber of receivers K in the solar field of the illustrative system 100is therefore K=NM. In other embodiments, the number of receivers mayvary from row to row.

Each receiver (e.g., receiver 108 a) may be either a conventionalreceiver constructed according to the prior art or a novel cavityreceiver according to embodiments of the invention as described herein.In other words, it is possible to build a system 100 using receivers ofthe present invention as new construction, or to retrofit some or all ofan existing system 100 with receivers of the present invention.

In one state of operation, a heat-transfer fluid (e.g., molten salt,refractory oil, a gas) is admitted to an entry manifold 116 by a valve118. In FIG. 1, any piping through which the heat-transfer fluid mayflow in some state of operation of the system 100 is denoted by a thickdark line. The heat-transfer fluid is distributed by the entry manifoldto the N rows and, in each row, passes sequentially through the Mreceivers in that row. In general, an approximately equal amount ofsolar radiation Q_(c) is gathered by the collector associated with eachreceiver, and absorbed by the fluid in each receiver with a collectorefficiency η_(r). Thus, an approximately equal quantity of solar energyQ_(r)=Q_(c)×η_(r) is added to the heat-transfer fluid during its passagethrough each receiver. In passing through the M receivers in each row,the heat-transfer fluid therefore gains an amount of energyapproximately equal to Q_(r)×M before entering an exit manifold 120. Thetotal amount of thermal energy Q_(total) added to the heat-transferfluid that enters through manifold 116, passes through the solar field102, and exits through manifold 120 is therefore, to a firstapproximation, Q_(t)=Q_(r)×M×N. However, in practice the amount ofenergy will be a lesser quantity Q′_(t)<Q_(t) because of losses fromcollectors and other components. Where the receivers arevacuum-containing tubes according to the prior art, receiver losses aredominated by, but not restricted to, radiation losses from collectors,i.e., infrared radiation emitted by the hot coating on each collectors'heat-absorbing element. Because emissive losses tend to increase byapproximately the fourth power of T, losses are greater from hottertubes, i.e., those closer to the exit manifold 120. Energy losses aretherefore not uniform throughout the solar field 102. The overallefficiency of the solar field, η_(f), depends complexly on direction andintensity of insolation (solar radiation impinging on the solar field102), collector efficiency, absorber coating absorptivity and emissivityas a function of temperature, receiver geometry, convection andconduction losses from receivers, heat-transfer fluid characteristics,and other factors.

Heated heat-transfer fluid leaving the solar field 102 through the exitmanifold 120 is gated by a valve 122. Two additional valves 124, 126direct the heat-transfer fluid from the exit manifold 120 to at leastone of a hot-fluid reservoir 128, piping 130, and piping 132. Fluid mayalso be extracted from hot reservoir 128 by a pump (not depicted).

Heat-transfer fluid passing through piping 132 encounters three devicesin which the fluid exchanges heat with liquid water or steam, i.e., asuperheater 134, a heater 136, and a preheater 138. In each of the threeheat-exchange devices 134, 136, 138, the heat-transfer fluid gives upsome of its thermal energy to liquid water or steam passing through thesame three devices 134, 136, 138 in the opposite direction. In FIG. 1,any piping through which water and/or steam may flow in some state ofoperation of the system 100 is denoted by a thick dashed line.Heat-transfer fluid passing through piping 130 passes through a fourthheat-exchange device (reheater) 140, giving up some of its thermalenergy to steam passing through the reheater 140 in the oppositedirection. Heat-transfer fluid that has flown through the heat-exchangedevices 134, 136, 1348, and 140 is re-united in piping 142. The fluidmay be directed through a valve 144 to a “cool” storage reservoir 146.Fluid may also be extracted from cool reservoir 146 by a pump (notdepicted). Heat-transfer fluid not directed to the cool reservoir 146passes through a pump 148 and is pressurized sufficiently to enablecirculation of the fluid through the solar field 102. Booster pumps forheat-transfer fluid and water and/or steam that are not depicted in FIG.1 may be present in the system 100. Valves 118 and 150 direct theheat-transfer fluid pressurized by the pump 148 to at least one of thesolar field 102 and a boiler 154. The boiler 154 may burn a fossil fuel(e.g., natural gas) or derive thermal energy from some other source inorder to heat the heat-transfer fluid passing therethrough. When thesolar field 102 is not capable of maintaining the temperature of theheat-transfer fluid throughout system 100 at or above an acceptableminimum (due to, e.g., lack of insolation), the boiler 152 may beemployed to heat the heat-transfer fluid. Alternatively or additionally,the boiler 152 may be used as a backup source of heat to drive energygeneration by the system 100.

Water heated in the preheater 138 is boiled to steam in the heater 136and the steam is further heated in the superheater 134. Steam from thesuperheater 134 is directed through piping 154 to a first turbine stage156. After expanding in the first turbine stage 156, imparting energythereto and declining in temperature, the steam is directed throughpiping 158 to the reheater 140, where it is heated bymaximum-temperature heat-transfer fluid direct from the solar field 102or from the hot-fluid reservoir 128. The steam is then directed throughpiping 160 to a second turbine stage 162. After expanding in the secondturbine stage 162, imparting energy thereto and declining intemperature, the steam is directed through piping 164 to a condenser166, where it gives up further energy (to, e.g., a preheater flow ofheat-transfer fluid not depicted in FIG. 1) and is condensed to water.The water from condensor 166 is directed through piping 168 to a waterreservoir 170. Water exiting the reservoir 170 passes through a pump172. Water pressurized by the pump 172 enters the initial heating loop(devices 138, 136, 134) and continues as already described.

The turbine stages 156, 162 turn a generator 174 that produceselectricity. In other embodiments, the heat or generated steam may beused for other processes such as industrial processes.

A controller (e.g., a general purpose programmable computer thatoperates under the control of one or more instructions recorded on amachine readable memory) 180 receives data from the solar field 102 (on,e.g., fluid temperature and pressure in different parts of the field;collector orientation; insolation intensity and direction) and sendscontrol signals to various parts of the solar field 102 to optimize itsimportance. For example, the controller may progressively tiltcollectors as the sun moves across the sky in order to collect maximumenergy.

In an ordinary state of energy-producing operation, the sole source ofenergy input to the system 100 is sunlight falling on the collectors ofthe solar field 102. The efficiency with which this solar energy isconverted to heat by absorption in the receivers (108 a, 110 a, . . . ,108 b, 110 b, . . . ), and retention of the energy thus absorbed by theheat-transfer fluid as it passes through the solar field 102, aretherefore influence the cost of electricity produced by the system 100.

FIG. 2 depicts an illustrative assembly 200 for the collection of solarenergy in the form of heat that may be part of a larger system for theconversion of solar energy to thermal and/or electrical energy. Assembly200 is typical of receiver-and-trough assemblies, according to the priorart, that could be used in the solar field 102 of a power-generatingsystem similar to that illustrated in FIG. 1. Receivers embodying theinvention could also be employed, advantageously, in assembly 200 and insolar field 102 in FIG. 1.

Illustrative assembly 200 comprises a receiver tube 202. Solar radiation204 is reflected from a trough-shape collector 206 having a paraboliccross-section and impinges on the receiver 202. The receiver 202 is heldin place at the focus of the parabolic trough 206 by struts 208. Theapproximately rigid assembly comprising receiver 202, struts 208, andtrough 206 is held in place by a truss-type support structure 210 whichis in turn mounted upon supports 212 and can be rotated on a joint orbearing mechanism 214 so that the trough 206 faces the sun (i.e., istilted at an angle equal to the elevation of the sun above the horizonat a given moment). Movement of the assembly 200 for sun-track, oroperation of other controllable features of the assembly 200, may becontrolled by a controller (e.g., a general purpose programmablecomputer that operates under the control of one or more instructionsrecorded on a machine readable memory) 216.

FIG. 3A (side view) and FIG. 3B (cross-sectional view) depict some ofthe features of a typical receiver tube 300 according to the prior art.The tube 300 is typical of tubes that would presently be used in thesolar field 102 of a power-generating system similar to that illustratedin FIG. 1. In receiver 300, a transparent glass envelope 302 contains asteel absorber tube (heat-absorbing element) 304. Between the envelope302 and the tube 304 is a vacuum (evacuated annulus) 306. An evacuationnozzle 308 permits removal of air from the space between the envelope302 and tube 304 during manufacture of the receiver 300. At each end ofthe receiver 300, a metal bellows 310 flexibly accommodates changingforces due to thermal expansion and contraction and to wind, andprovides a point of attachment for the struts (e.g., struts 208 in FIG.2) that support the receiver 300. A glass-to-metal seal 312 preservesthe vacuum 306. A “getter” insert 314 absorbs some of the hydrogen thatinfiltrates the vacuum 306.

FIG. 3B makes clear that the tube 304 is coaxial with the envelope 302,with the vacuum 306 between them. In various embodiments according tothe prior art, coatings to selectively absorb and reflect radiation,conduct heat, resist corrosion, or achieve other purposes are applied tothe outer surface 318 and inner surface 320 of the envelope 302 and tothe outer surface 322 and inner surface 324 of the tube 304.Heat-transfer fluid 316 circulates within the tube 304.

FIG. 4 pertains to one embodiment of the invention. Before describingFIG. 4 in detail, we note the following general considerations, whichpertain to various embodiments of the invention: Various embodiments,including that depicted in FIG. 4, comprise a tubular heat-absorbingelement partly enclosed in an insulating layer or jacket, with a recessor cavity (also herein termed the “optical cavity”), transparent tolight on one side along an aperture, below an exposed surface (alsotermed “the absorbing surface”) of the heat-absorbing element. Incross-section, the insulation jacket has approximately the form of aletter “C” that can be turned, or is permanently turned, so that theopening of the “C” faces toward a parabolic collector that focuses lightthereon. In this analogy, the gap in the “C” corresponds to the opticalcavity, and the width of the gap in the “C” corresponds roughly to thewidth of the receiver aperture. A portion of the heat-absorbing element,herein termed the “absorbing surface,” is exposed to light entering theoptical cavity through the aperture. Making the absorbing surface planar(i.e., flat) rather than another shape (e.g., an arc) reduces theabsorbing surface's area for a given aperture width; the advantagesrealized by doing so are discussed further below.

An additional design consideration in various embodiments is theinclusion or omission of a transparent aperture cover (e.g., of glass)enclosing the recessed optical cavity. Omitting the aperture cover tendsto improve optical performance but hinder thermal performance in somecircumstances (e.g., wind directed along cavity) and tends to expose theabsorber surface to potential contamination (e.g., by dust). Atransparent aperture cover may be shaped to act as a lens thatconcentrates light that is incident on the aperture (especially lightincident on the aperture at a low angle) into a narrower cavity. Alensing aperture cover can enable further reduction in absorbing-surfacesize, as clarified in later Figures and descriptions.

An additional design consideration in various embodiments is theinclusion or omission of reflective parabolic sidewalls that bound thelateral faces of the recessed optical cavity. Such reflectors, alsoherein termed “compound parabolic concentrators” because they constitutea stage of parabolic concentration additional to that of the collectortrough, can enable further reduction in absorbing-surface size, aperturewidth, and overall receiver width, as clarified in later Figures anddescriptions. If compound parabolic concentrators are omitted, thesidewalls of the optical cavity may be oriented and shaped so as tointercept essentially none of the light-rays directed into the opticalcavity by the collector, with or without passage through a lensingaperture cover. Such non-intercepting sidewalls are also herein termed“passive” sidewalls. Reflective sidewalls having non-parabolicgeometries may also be employed.

Receiver designs incorporating (a) a tubular heat-absorbing elementpartially surrounded by an insulating jacket, (b) a recessed opticalcavity with sidewalls (either passive sidewalls or compound parabolicconcentrators), and (c) a portion of the heat-absorbing element that isexposed to light in at least some states of operation of the receiverand which, when exposed to light, acts as absorbing surface (eitherplanar or curved), are herein also referred to as illustrative of the“advanced cavity receiver geometry.” Various embodiments of the advancedcavity receiver geometry may also include one or more of the followingfeatures, among others: (a) a tubular shell around the insulation layer,(b) an outermost tubular cover that may be moveable, or with respect towhich the other components of the receiver may be moveable, and whichmay incorporate transparent or insulating covers that can cover theaperture in various states of operation, (c) gas (e.g., air) within theoptical cavity and/or other portions of the receiver, which gas may beat atmospheric pressure, (d) a vacuum within the optical cavity and/orother portions of the receiver, and (e) a transparent cover over part orall of the aperture, which cover may be shaped to act as a lens, (f)various specialized optical and thermal coatings over the surfaces ofany of the components (e.g., over a transparent aperture cover), (g) thecapability to flow/insert an inert gas into the aperture.

The geometric details of various embodiments of the advanced cavityreceiver, particular the depth of the optical cavity and the angles andshapes of the sidewalls of the optical cavity, are influenced by thecollector rim angle (this term is clarified in FIG. 10). Variations onthe advanced cavity receiver design seek to minimize absorbing-surfacewidth and overall cavity width, to maintain sufficient cavity depth forinsulating purposes, and to operate efficiently in conjunction eitherwith existing trough collectors or new collectors having similar keydimensions.

FIG. 4 is a cross-sectional diagram that depicts features of anillustrative novel receiver that incorporates aspects of the inventionand shows one possible response to the range of design considerationsdiscussed in the foregoing paragraphs. In this novel design, an advancedcavity receiver 400, is a dramatic departure from the construction ofprior art receivers (e.g., as depicted in FIG. 3A and FIG. 3B). Thisillustrative advanced cavity receiver is simpler to manufacture andmaintain than the conventional receiver of FIG. 3B because it does notcontain a vacuum and reduces the exposed glass component. Elimination ofthe vacuum confers a number of advantages, including obviation of theairtight seal 312 in FIG. 3 and the “getter” insert 314, elimination ofvacuum failure (responsible for failure of ˜1-5% of conventionalvacuum-containing receivers per year in real-world usage), and possibleeasing of design constraints on the bellows 310 or equivalentcontrivances for accommodating structural stresses and movements (sincethere is no airtight seal that needs to be protected against mechanicalfailure). In additional, the removal of vacuum baffles allows foradjacent receivers to be nearly continuous.

The illustrative advanced cavity receiver 400 comprises a tubular shell402, a heat-absorbing element 404 through which a heat-transfer fluid406 can flow, reflective parabolic sidewalls 408, 410, an aperture 412,a transparent aperture cover 414 spanning the aperture 412, aninsulating jacket 416 partially surrounding the heat-absorbing element404, a planar (flat) absorbing surface 418 coated with an absorbentcoating and exposed to light entering the aperture 412, and an opticalcavity 420 bounded by the absorbing surface 418, sidewalls 408, 410, andaperture cover 414. The shell 402 and insulation 416 may besubstantially opaque or be optical transmitting. Substantially opaqueinsulation blocks most but not necessarily all of the radiation. Opticaltransmitting insulation has a transmittance of above 80%. The receiver400 is approximately uniform in cross-section along its entire length,apart from mounting and other hardware (not shown) at each end. Theoptical cavity or other portions of the collector 400 may be evacuatedor filled with a gas (e.g., air) at approximately ambient atmosphericpressure. If the gas is air, the advantage is gained over the prior artthat no provision need be made to exclude ambient air from any portionof the receiver 400 except the fluid-containing heat-absorbing element404.

The specific dimensions of the components of a receiver having aschematic cross-section like receiver 400 are chosen to optimizeperformance (i.e., solar energy collected) when the receiver 400 ismounted at the focus of a trough-shaped collector of specific dimensionswhose reflective surface is parabolic in cross-section. That is, thedesign of a receiver cannot be considered entirely apart from thedimensions of the collector with which it is intended to work. Geometricand other considerations pertaining to the design of a receiverresembling receiver 400 in FIG. 4 are described and clarified further inFIG. 18 and other Figures below.

In various embodiments, the selective absorber coating on the absorbingsurface 418 is a spectrally selective plasmonic nickel nanochain cermet(ceramic-metal blend) material that may be created usingsolution-chemical fabrication. The advantages offered by advancedcoating materials of this type include, but are not limited to, simplerfabrication than conventional cermet absorber coatings, tolerance ofhigher temperatures, and desirable absorbance and emissivity properties.Such materials have been demonstrated in the laboratory by Prof JifengLiu and colleagues at Dartmouth College, NH and disclosed in, e.g., WangX, Li H, Yu X, Shi X, and Liu J, “High-performance solution-processedplasmonic Ni nanochain-Al₂O₃ selective solar thermal absorbers,” Appl.Phys. Lett. 101, 203109 (2012), but have not hitherto been disclosed aspart of any novel receiver design such as the advanced cavity receiver400.

In brief, receiver 400 operates as follows: light rays 422, 424 arefocused across a range of angles upon the aperture 412 by a paraboliccollector (not shown). All light rays passing through the transparentaperture cover 414 strike the absorber surface 418 after either (a)traversing the optical cavity 420 directly, as does ray 424, or (b)reflecting off a parabolic sidewall 408 or 410, as does ray 422. Theabsorber surface 418 absorbs a large portion of the solar radiationincident upon it and is heated thereby. Because the absorber surface 418is planar, in a departure from the prior art, it minimizes the surfacearea over which infrared light is emitted by the heated absorbercoating. Any non-planar absorber surface in a receiver otherwiseidentical to receiver 400 would, by straightforward geometry, have alarger surface area than the planar absorber surface 418. Consequently,for a given coating composition at a given temperature, the planarabsorber surface of FIG. 4 will radiate less energy than a non-planarabsorber surface simply because its area is smaller. In an extreme case,the absorber coating of a receiver constructed according to the priorart is 100% of the lateral surface of the tubular heat-absorbingelement. By contrast, in various embodiments incorporating aspects ofthe invention, the absorbing surface (e.g., absorbing surface 418 inFIG. 4) may occupy only 20% of the lateral surface area of theheat-absorbing element, and at most 50% of the lateral surface of theheat-absorbing element.

Moreover, the receiver geometry and advanced coatings proposed for theabsorber surface 418 will allow operation of the receiver 400 at highertemperatures than can be tolerated by the absorber coatings ofvacuum-tube receivers constructed according to the prior art. Operationat higher temperature (i.e., temperature of absorber surface 418,heat-absorbing element 404, and heat-transfer fluid 406) inenergy-generating facility such as system 100 in FIG. 1 will, ingeneral, operate more efficiently if the temperature of theheat-transfer fluid exiting the solar field 102 is higher: preferably,as high as the material construction of the system 100 permits.

These and other advantages of the invention will be made clear insubsequent Figures and descriptions.

In order to operate a solar field 102 at elevated temperatures, it isdesirable that heat-transfer fluid be available which can tolerate suchelevated temperatures and have other advantageous properties (e.g., doesnot freeze ambient environmental temperatures). Candidate heat-transferfluids include, e.g., molten solar salt, steam, superheated steam,high-pressure steam, Coastal Chemical's HITEC® heat transfer salt(“HITEC Heat Transfer Salt,” Coast Chemical Co., LLC,www.coal2nuclear.com/MSR%20-%20HITEC%20Heat%20Transfer%20Salt.pdf,accessed Jan. 6, 2012), Sandia National Laboratory's low-melting-pointinorganic nitrate salt fluid (“Low-melting point inorganic nitrate saltheat transfer fluid,” U.S. Pat. No. 7,588,694, issued Sep. 15, 2009),the hybrid organic siloxane-based fluid being developed by Los AlamosNational Laboratory (“Hybrid Organic Silicone HTF Utilizing EndothermicChemical Reactions for Latent Heat Storage,”http://www1.eere.energy.gov/solar/sunshot/csp_recoveryact_lan1.html,accessed Jan. 6, 2012), and CX500, a molecular silicone-based fluid withexceptional thermal stability (to at least 500° C.) and low freezingpoint (−40° C.) developed at Los Alamos National Laboratory by Dr.Stephen Obrey and colleagues (P. C. DeBurgomaster, S. J. Obrey, L. N.Lopez, “Rational Tailoring of Functional Siloxanes for Enhanced ThermalPerformance,” Presentation at the American Chemical Society NationalMeeting in Anaheim, Calif., 2011, athttp://permalink.lan1.gov/object/tr?what=info:lan1-repo/lareport/LA-UR-11-01885,accessed Jan. 6, 2012).

FIG. 5A includes two data plots, Plot 5A and Plot 5B, which compareaspects of performance of a state-of-the-art conventional receiver(i.e., the Schott PTR70) and of an illustrative advanced cavity receiverthat resembles receiver 400 in FIG. 4 and incorporates aspects of theinvention. Figures for Schott PTR70 conventional receiver performanceare derived from Burkholder F, Kutscher C, “Heat Loss Testing ofSchott's 2008 PTR70 Parabolic Trough Receiver,” National RenewableEnergy Laboratory (NREL) Technical Report NREL/TP-550-45633, May 2009.Testing by Burkholder and Kutscher did not exceed 500° C., and thecermet absorber coating of the Schott PTR70 cannot endure prolongedexposure to a temperature of 650° C.; therefore, the efficiency figuresfor “Current Receivers” in Plot 5A and Plot 5B are based onextrapolation of curves presented by Burkholder and Kutchscher (2009)(whose results are collectively henceforward referred to as “NRELdata”). The figures represent the approximate performance that astate-of-the-art conventional receiver would achieve if it were endowedwith an absorber coating capable of operating at temperatures aboveapproximately 500° C.

To predict performance of advanced cavity receiver designs, a computermodel was constructed for both the conventional receiver design and arange of advanced cavity designs. Validation of the model, andadditional results therefrom, will be further explained in laterFigures.

The Zemax software tool was used to predict the optical efficiency ofboth the conventional and advanced cavity receiver. Zemax is availablefrom Radiant Zemax, 22908 NE Alder Crest Drive, Suite 100, Redmond,Wash. 98053. The ANSYS Fluent software package was used to predict thethermal efficiency of both the conventional receiver and a range ofadvanced cavity receivers. ANSYS Fluent is available from ANSYS, Inc.,275 Technology Drive, Canonsburg, Pa. 15317.

As used herein, the “optical efficiency” of a receiver is defined as thefraction of the radiant energy incident on the collector that iscollected as heat by the heat-transfer fluid within the receiver.Simulation of the conventional receiver was compared to NREL data tovalidate the model: conventional-receiver results matched both measuredand extrapolated results within 1-3%, lending confidence to theadvanced-cavity receiver results presented herein.

As used herein, the “total efficiency” of a receiver is the fraction ofradiant energy focused upon the receiver that is delivered as heatenergy by the heat-transfer fluid upon exit from the receiver, and isdefined as the product of the receiver's optical efficiency and thermalefficiency.

As used herein, the “thermal efficiency” of the receiver is defined asone minus the heat loss from the receiver divided by the energy absorbedby the receiver. In computer simulations of receiver behavior describedherein, to specify the thermal efficiency independently of the opticalefficiency, the absorbed energy is kept constant at 4500 W/m. This valueassumes an optical efficiency of 90% for an incident solar energy sourceof 1000 W/m² on a collector with a 5 m diameter. Heat-loss data for thestate-of-the-art Schott PTR-70 receiver have been experimentallydetermined (the NREL data) or are extrapolated from such data, whileheat loss for other receivers is calculated herein using computationalfluid dynamics models. Table 1 shows that planar receivers (e.g.,receiver 400 of FIG. 4) offer increased thermal efficiency performancerelative to a state-of-the-art receiver. The numbers given in Table 1are clarified below. As used herein planar is used to generallydistinguish a structure of the present invention from a strictlycircular structure and as such includes variations that include somedegree of curvature. For example in some embodiments to facilitate theefficient transport of fluid the central fluid transport channel caninclude rounded corners and a generally bowed cross-sectional profile

TABLE 1 Thermal Efficiency of Schott PTR70 and Planar Receiver Schott-Modeled PTR70 Planar Operating Receiver Efficiency Efficiency EfficiencyReceiver Temperature Prior Art Estimate 1 Estimate 2 Estimate 3Efficiency 400 94.9% 95.5% 96% 96.1% 96.8% 450 92.6% 93.8% 94% 94.4%96.2% 500 89.4% 91.4% 92% 92.4% 95.3% 550 85.1% 88.2% 89% 90.2% 94.3%600 79.3% 83.9% 85% 86.8% 93.1% 650 71.7% 78.4% 80% 81.7% 91.7%

While efforts have been made to include relevant important physicalfactors in the CFD models, it is likely that various embodiments of theinvention will have efficiencies less than those calculated by the idealmodel value. The thermal efficiencies of these embodiments may fallbetween those of the modeled planar receiver and the measured PTR70receiver due to additional heat loss mechanisms and unaccounted forphysical factors. Three estimates for these efficiencies are thefollowing:

The estimate 1 percentage is defined as:

${{Estimate}\mspace{14mu} 1\%} = {{{SOA}\mspace{14mu} \%} + \frac{{{Opimized}\mspace{14mu} {Planar}\mspace{14mu} {Receiver}\mspace{14mu} \%} - {{SOA}\mspace{14mu} \%}}{3}}$

Estimate 2 is the average of estimate 1 and estimate 3, expressed as thenearest whole number.

The estimate 3 percentage is defined as:

${{Estimate}\mspace{14mu} 3\%} = \frac{{{Opimized}\mspace{14mu} {Planar}\mspace{14mu} {Receiver}\mspace{14mu} \%} + {{SOA}\mspace{14mu} \%}}{2}$

Plot 5A in FIG. 5A shows that in at least one embodiment, thevacuum-free advanced cavity receiver maintains >90% receiver thermalefficiency (plotted using black squares and solid lines) up to 650° C.operating temperature, substantially outperforming the currentvacuum-based state-of-the-art receiver (plotted using black diamonds anddotted lines), whose thermal efficiency drops below 75% efficiency at650° C.

Plot 5B in FIG. 5A combines the thermal-efficiency curves of Plot 5Awith optical-efficiency curves (not shown) to derive total receiverefficiency curves for the same current vacuum-based receiver andadvanced cavity receiver as in Plot 5A. Even with operation in air andthe complete removal of receiver vacuum, performance of the advancedcavity receiver at current operating temperatures (˜375° C.) isapproximately equal to that of the current receiver; at highertemperatures (i.e., above approximately 475° C.), the total receiver ofthe efficiency of the advanced cavity receiver exceeds that of thecurrent receiver.

Moreover, the estimated materials and manufacturing cost of theembodiment whose simulated performance is depicted in FIG. 5A areapproximately 20% lower than those of the state-of-the-art receiverwhose performance is depicted in FIG. 5A. Therefore, advantages to berealized by at least some embodiments of the invention include higherefficiency at high operating temperatures and lower capital cost perreceiver. Lower rates of replacement for receivers in the field isanother advantage likely to be realized by at least some embodiments ofthe invention, given the complete absence of vacuum degradation as aconcern in the advanced cavity receiver.

FIG. 5B is a cross-sectional plot of simulated contours of constanttemperature (degrees Centigrade) in a state-of-the-art receiver 500constructed according to the prior art and operating at a fixedtemperature of the absorber coating of 400° C. Simulation was performedusing the ANSYS Fluent software tool, which was also used to create allother plots of simulated contours of constant temperature and ofvelocity vectors in circulating gases in subsequent Figures. In thisinstance, the vacuum normally present in a state-of-the-art receiver 500between its envelope 502 and heat-absorbing element 504 is presumed tohave been lost, i.e., the space between the envelope 502 andheat-absorbing element 504 is filled with air. This simulation alsoassumes a heat-absorbing coating having an emissivity of approximately0.1 that does not degrade when exposed to air i.e., does not increase inemissivity. As used herein, the emissivity of an object is thedimensionless ratio of the energy radiated by a the material to theenergy radiated by an ideal black body at the same temperature. Assuminga nondegraded coating is a conservative assumption, because actualcoatings according to the previous art do degrade when exposed to airand increase in emissivity. Increased emissivity entails increasedenergy loss through infrared radiation from the heat-absorbing surface.

FIG. 5C is a plot of simulated radiative heat loss in units of watts permeter for the same simulated state-of-the-art receiver 500 depicted incross-section in FIG. 5B. Simulated heat loss for a range of assumedconditions and operating temperatures is plotted in FIG. 5C. For each ofthree operating temperatures (i.e., 400° C., 550° C., 650° C.), thethree assumed operating conditions are as follows: (1) The black barscorrespond to a receiver 500 with intact vacuum; (2) thehorizontal-striped bars correspond to a receiver 500 with vacuum lost(as in FIG. 5B) but no degradation of absorber coating (i.e., noincrease in emissivity above 0.1); and (3) the diagonal-striped barscorrespond to a receiver 500 with vacuum lost (as in FIG. 5B) and withrealistic degradation of absorber coating (i.e., emissivity increased to0.65). The operating condition and temperature of the contour plot ofFIG. 5B corresponds to the horizontal striped bar in FIG. 5C for anoperating temperature of 400° C. FIG. 5C shows that radiative heat lossincreases both with temperature increase and with loss of vacuum; thisclarifies (a) the disadvantage conferred on the prior art by reliance onvacuum and (b) the even more drastic disadvantage conferred on the priorart by coatings that increase in emissivity when exposed to air and tohigher temperatures (e.g., 550° C. and 600° C.).

FIG. 6 pertains to the validation of the ANSYS Fluent computationalfluid dynamics numerical (computer) model used to predict theperformance of a range of novel, advanced cavity receiver designs,including that whose performance is depicted in FIG. 5A and FIG. 5B.Thermal performance of receiver configurations was determined byanalyzing the results of radiative-loss, convective-loss, andconductive-loss models in ANSYS Fluent, a comprehensive finite-volumecomputational fluid dynamics program. Radiative heat transfer losseswere solved for using the discrete-ordinates radiation model, in whichthe spatial domain is discretized into a finite number of directions andthe model solves the radiative transfer equation for each discretizeddirection. The radiation, conduction, and convective heat transfermodels are coupled in the Fluent solver through the energy equation andthe core Navier-Stokes equations. Fluent can capture the effects ofmaterial properties such as surface emissivity and temperature-dependentgas behavior.

Models to study natural convection effects within air-filled receiverdesigns were tested by comparing to NREL data. The receiver thermalmodels are based on a two-dimensional cross section of each receiverwith a constant-temperature boundary condition at the external surfaceof the heat-absorbing element. This boundary condition setup reflectsparabolic-trough plant operation for a target heat-transfer fluid loopoutlet temperature.

The computational fluid dynamics model for the current state-of-the-artreceiver is dominated by heat losses due to radiation from the outersurface of the heat-absorbing element; convective and conductive lossesthrough the vacuum to the envelope (and thence to the environment) arenegligible. To test the model's realism, the model was first used topredict the heat loss (characterized in watts per meter of receiver) ofa conventional vacuum-based receiver (i.e., the Schott PTR70)independently of that receiver's performance as measured andextrapolated in the NREL data. Model values (plotted as black squares)in FIG. 6 match experimental and extrapolated NREL data (plotted asblack diamonds in FIG. 6) within 1% for 400° C. and 550° C., and within3% for 650° C. A convection-only model (omitting radiation) matched heatlosses predicted from the Raithby and Holland annular flow heat-transfercorrelation within 10% (see Raithby and Holland, “Natural Convection,”Ch. 4, Handbook of Heat Transfer, Rohsenow W M, Hartnett J P, Cho Y Ieds., McGraw-Hill, 1998), which is acceptable given the uncertainty inexperimental correlations. As further assurance, our model of aconventional receiver with lost vacuum matched NREL data within 4%.

FIG. 7 cross-sectionally depicts an illustrative receiver thatincorporates features of the invention. The illustrative advanced cavityreceiver 700 comprises a tubular shell 702, a heat-absorbing element 704through which a heat-transfer fluid 706 can flow, planar sidewalls 708,710, an aperture 712, a transparent aperture cover 714 made of glass oranother transparent material and spanning the aperture 712, aninsulating jacket 716 partially surrounding the heat-absorbing element704, a substantially planar (flat) absorbing surface 718 coated with anradiation-absorbent coating and exposed to light that enters theaperture 712, and an optical cavity 720 bounded by the absorbing surface718, sidewalls 708, 710, and aperture cover 714. In other embodiments,the aperture cover 714 could be a partial covering to reducetransmission losses (e.g., see FIG. 30), or could be absent. Thereceiver 700 is approximately uniform in cross-section along its entirelength, apart from mounting and other hardware (not shown) at each end.The optical cavity 720 and other portions of the collector 700 arefilled with a gas (e.g., air) at approximately ambient atmosphericpressure.

The receiver cavity sidewalls 708, 710 are sloped at an angle chosen tomaximize admission of radiation from a reflective parabolic troughcollector (not depicted). Light rays concentrated by the collector(e.g., rays 722, 724) enter the aperture 712 and impinge directly onabsorbing surface 718. The insulation may be substantially opaque or beoptically transmitting.

Thickness and properties of the insulation 716, along with thedimensions of the shell 702, are chosen as a compromise betweenminimizing thermal losses and minimizing shadowing of the collector bythe receiver. The insulating material maintains low thermal conductivityat high temperatures (>600° C.). The radiation-absorbent coating on theheat-absorbing element 704 has high (e.g., >90%) absorptance and low(e.g., <15%) thermal emittance and is stable at receiver operatingtemperatures and under indefinite exposure to ambient conditions (air,humidity). The transparent aperture cover 714 may include a coating onits external surface 726 to mitigate reflection of light from the cover714.

FIG. 8 is a cross-sectional plot of simulated contours of constanttemperature (degrees Centigrade) in three advanced cavity receivers 800,802, 804 that embody aspects of the invention. Receivers 800, 802, and804 all geometrically resemble the illustrative advanced cavity receiverdepicted in FIG. 7: i.e., all three receivers 800, 802, 804 comprise atubular shell 806, 808, 810, a heat-absorbing element 812, 814, 816through which a heat-transfer fluid 816, 820, 822 can flow,substantially planar sidewalls 824, 826, 828, 830, 832, 834, an aperture836, 838, 840, a transparent aperture cover 842, 844, 846 spanning theaperture 836, 838, 840, an insulating jacket 848, 850, 852 partiallysurrounding the heat-absorbing element 812, 814, 816, a substantiallyplanar (flat) absorbing surface (absorber) 854, 856, 858 coated with anradiation-absorbent coating and exposed to light that enters theaperture 836, 838, 840, and an optical cavity 860, 862, 864 bounded bythe absorbing surface 854, 856, 858, sidewalls 824, 826, 828, 830, 832,834, and aperture cover 842, 844, 846 made of glass. Each receiver 800,802, 804 is approximately uniform in cross-section along its entirelength, apart from mounting and other hardware (not shown) at each end.The optical cavity 860, 862, 864 of each collector 800, 802, 804 isfilled with air at atmospheric pressure.

The three receivers 800, 802, and 804 differ in the dimensions ofcertain components and are not necessarily drawn to a common scale inFIG. 8. In particular, receiver 800 has an absorber 854 that is 6 cmwide and an insulating jacket 848 that is 2.25 cm thick (measured atright angles from any point on the lateral or upper surfaces of theheat-absorbing element 818); receiver 802 has an absorber 856 that is 5cm wide and an insulating jacket 850 that is 3.0 cm thick (measured asfor receiver 800); and receiver 804 has an absorber 858 that is 5 cmwide and an insulating jacket 852 that is 3.5 cm thick (measured atright angles from any point on the lateral or upper surfaces of theheat-absorbing element 818).

Receiver 800 is simulated as operating at 400° C. (fixed temperature ofthe absorber coating), receiver 802 is simulated at operating at 550°C., and receiver 804 is simulated as operating at 650° C. FIG. 8 showsresults from a ANSYS Fluent computational model of receiver thermalproperties as described above in reference to FIG. 5A and FIG. 5B.

The constant-temperature contours in FIG. 8 show that, as would beexpected from elementary physics, in all three receivers 800, 802, 804,the surface temperature of each receiver is lower over the surface ofthe insulating jacket 848, 850, 852 than at the aperture cover 842, 844,846, the latter being separated from the heat-absorbing surface 854,856, 858 only by air. Although the surface area of the non-aperturesurface of each receiver 800, 802, 804 is greater than that of theaperture cover 842, 844, 846, the temperature at the aperture cover 842,844, 846, is so much greater than that over the surface of theinsulating jacket 848, 850, 852 that most loss occurs in all designsprimarily through the aperture.

FIG. 9 is a plot of measured and simulated heat loss (watts per meter)for 10 receiver designs, namely a state-of-the-art (SOA) receiverconstructed according to the prior art (i.e., the Schott PTR70) and nineillustrative advanced cavity receivers resembling the receiver 700depicted in FIG. 7 and the receivers 800, 802, and 804 in FIG. 8. Thenine illustrative advanced cavity receivers differ in width of absorbingsurface and thickness of insulating jacket, as follows:

Design 1 has absorber width 5 cm and insulating jacket thickness 2.25cm.

Design 2 has absorber width 6 cm and insulating jacket thickness 2.25cm.

Design 3 has absorber width 7 cm and insulating jacket thickness 2.25cm.

Designs 1-3 are simulated at 400° C. for FIG. 9.

Design 4 has absorber width 5 cm and insulating jacket thickness 3.0 cm.

Design 5 has absorber width 6 cm and insulating jacket thickness 3.0 cm.

Design 6 has absorber width 7 cm and insulating jacket thickness 3.0 cm.

Designs 4-6 are simulated at 550° C. for FIG. 9.

Design 7 has absorber width 5 cm and insulating jacket thickness 3.5 cm.

Design 8 has absorber width 6 cm and insulating jacket thickness 3.5 cm.

Design 9 has absorber width 7 cm and insulating jacket thickness 3.5 cm.

Designs 7-9 are simulated at 650° C. for FIG. 9.

The simulation condition for receiver 800 in FIG. 8 corresponds to thesimulation of Design 2 at 400° C. for FIG. 9; the simulation conditionfor receiver 802 in FIG. 8 corresponds to the simulation of Design 4 at550° C. for FIG. 9; and the simulation condition for receiver 804 inFIG. 8 corresponds to the simulation of Design 7 at 650° C. for FIG. 9.

NREL data (i.e., Burkholder and Kutscher 2008, described above for FIG.5A) for the Schott PTR70 did not exceed 500° C., and the cermet absorbercoating of the Schott PTR70 cannot endure prolonged exposure to atemperature of 650° C.; therefore, the heat-loss figures for “SOA”receiver in FIG. 9 for temperatures above 500° C. are based onextrapolation of the NREL data, making the assumption that the cermetabsorber coating of the SOA receiver remains stable at such hightemperatures. In other words, actual SOA performance would be worse at550° C. and 650° C. than is depicted in FIG. 9. Figures for the threeadvanced cavity receivers at all temperatures are based on the ANSYSFluent simulation tool described above for FIG. 5A and FIG. 6.

FIG. 9 shows that the three illustrative advanced cavity receiversexperience lower thermal losses than the SOA receiver at all operatingtemperatures examined, and that the cavity receiver losses increase muchmore slowly with temperature than those of the SOA receiver. Also,increasing the width of the heat-absorbing surface (absorber) increasesthermal losses (albeit modestly) across the three advanced-cavityreceiver designs (5 cm, 6 cm, 7 cm absorber width). This is as expected,since increased absorber area increases the area for energy loss throughinfrared radiation. FIG. 9 thus also illustrates the advantage, ingeneral, of decreasing absorber area (e.g., through use of a planarrather than a curved heat-absorbing surface of given width).

Another design consideration relevant to the nine advanced cavityreceiver designs simulated for FIG. 9 is that there is a tradeoffbetween increasing insulation thickness and decreasing overallefficiency. Increasing insulation thickness decreases thermal losses,but increases shadowing of the collector by the receiver: e.g., for acollector having a 5 m aperture, each addition cm of insulationthickness on each side of the heat-absorbing element will increaseshadowing of the collector by approximately 0.2%, and so reduce systemoptical efficiency by approximately 0.2%. In general, the optimuminsulation thickness will be that which maximizes overall energeticefficiency of the collection system.

FIG. 10 illustrates terms used herein as they pertain to a receiver 1000mounted above a trough mirror 1002 having a uniform, paraboliccross-section and parallel to the receiver 1000. The collector “aperturelength” is the length of a line segment perpendicular to the collector1000 and connecting the rims of the mirror 1002. The collector “focallength” is the length of a line segment 1006 connecting the vertex 1008of the parabola to the focus 1010 of the parabola. The receiver 1000 islocated at or near the focus 1010. The “rim angle” 1012 of the collectoris the angle between line segment 1006 and a second line segment 1014perpendicular to the axis of the receiver 1000 and connecting the focus1010 to a point 1016 on the rim of the collector 1002. The rim angleextent is the same on either side of bisecting line 1006.

Gossamer's extremely low angular error derives from the novelspace-frame technology that the company has perfected. Per NREL/Sandiatesting, Gossamer's standard 7.3-m trough, which is commercially ready,offers up to 99.3% intercept factor on a PTR70 (70 mm receiver tube),where standard designs are usually <96%), and an unprecedented 104×concentration factor.

In addition to the support offered by angular accuracy to a smallercavity opening, a flatter reflector profile may enable designs that aremore rigid and less susceptible to torsional errors and winddisturbances. Some studies suggest that wind-induced imperfections are asignificant factor, and that a more planar structure may be of benefit17. In general, the design of our cavity opening will exploit recentimpressive advances in collector optical error (i.e., Gossamer at <2mrad).

FIG. 11 cross-sectionally illustrates an illustrative advanced cavityreceiver 1100 embodying aspects of the invention. The illustrativeadvanced cavity receiver 1100 comprises a tubular shell 1102, aheat-absorbing element 1104 through which a heat-transfer fluid 1106 canflow, planar sidewalls 1108, 1110, an aperture 1112, a transparentaperture cover 1114 spanning the aperture 1112, an insulating jacket1116 partially surrounding the heat-absorbing element 1104, asubstantially planar (flat) absorbing surface 1118 coated with anradiation-absorbent coating and exposed to light that enters theaperture 1112, and an optical cavity 1120 bounded by the absorbingsurface 1118, sidewalls 1108, 1110, and aperture cover 1114. Thereceiver 1100 is approximately uniform in cross-section along its entirelength, apart from mounting and other hardware (not shown) at each end.The optical cavity 1120 and other portions of the collector 1100 arefilled with a gas (e.g., air) at approximately ambient atmosphericpressure.

With a planar aperture cover (e.g., cover 714 in FIG. 7) having notendency to refract incident light, the receiver 1100 would becompatible with a first rim angle 1124: that is, light rays incident ata rim angle greater than that defined by the sidewalls 1108, 1110 wouldnot travel directly to the absorbing surface 1118 (although they might,in various embodiments, be absorbed and re-radiated by a sidewall 1108,1110 or be reflected therefrom to the absorbing surface 1118). Theaperture cover 1114 is lenticular in cross-section, having a planarupper surface and a convex lower surface. Light directed upon on theaperture cover 1114 by a parabolic collector (not shown) is refracted bythe aperture cover 114 toward the absorbing surface 1118. An exemplarylight ray 1122 shows how the lenticular aperture 1114 directs lighttoward the absorbing surface 1118. With a lenticular aperture cover1114, the receiver 1100 is compatible with a second rim angle 1126 thatis larger than the rim angle 1124 that would be compatible with asimilar receiver equipped, instead, with a planar aperture cover. Thatis, light rays incident at a rim angle 1126 greater than that defined bythe sidewalls 1108, 1110 are collected by the lenticular aperture cover1114 and directed to the absorbing surface 1118. One advantage conferredby the inclusion of a lenticular aperture cover 1114 is that for a givenrim angle (i.e., collector geometry), a collector 1100 having anarrower, deeper optical cavity 1120 and thus, in general, lower thermallosses will be feasible. In various embodiments, to reduce thermallosses it is in general advantageous to minimize absorber width andoptical cavity width.

FIG. 12 cross-sectionally illustrates an illustrative advanced cavityreceiver 1200 embodying features of the invention. Receiver 1200resembles receiver 1100 in FIG. 11, but includes a lenticular aperturecover 1202 having a different cross-sectional shape. The lenticularaperture cover 1202 has a concave upper surface and a convex lowersurface. If equipped with a planar aperture cover, receiver 1200 wouldaccept light from a maximum rim angle 1204; if equipped with alenticular aperture cover resembling that in FIG. 11, receiver 1200would accept light from a greater maximum rim angle 1206; with thelenticular aperture cover shape depicted in FIG. 12, receiver 1200 wouldaccept light from a yet greater maximum rim angle 1208. FIG. 11 and FIG.12 illustrate how lenticular aperture-cover geometry can influenceoverall collector geometry and thus collector efficiency.

FIG. 13 depicts Zemax simulations of a receiver/collector geometrytypical of a state-of-the-art receiver (e.g., the Schott PTR70). For thereader's convenience, the upper portion of FIG. 13 (FIG. 13A, FIG. 13Band FIG. 13C) shows the geometry without numerical labels. The lowerportion of FIG. 13 (FIG. 13D, FIG. 13E and FIG. 13F) shows the geometrywith numerical labels. Reference is now made to the lower portion ofFIG. 13.

The left-hand panel (FIG. 13D) shows how parallel light rays 1302 from asolar source are focused by a parabolic collector 1304 onto a receivertube 1306. In this figure, the source is modeled as having a spatiallyhomogeneous distribution and a uniform angular distribution within a4.7-mrad-radius disk, corresponding to the angular extent of the Sun. Ina magnified view of the receiver (FIG. 13E), reflected rays from thecollector 1304 are shown to strike the receiver envelope directly 1308,be refracted by the envelope glass 1310 and strike the absorbing tube ofthe receiver 1312. One notable ray 1314 strikes the receiver envelopedirectly from the solar source, is refracted by the envelope glass andescapes the receiver. The furthest right panel (FIG. 13F) depicts thecomplex interaction between a single light ray and the receivermaterials. Here, a single ray 1316 incident from the collector resultsin many subsequent rays due to surface interactions due to refraction,partial reflection 1320 and scattering 1322 (e.g., by the absorbingsurface 1318. In this model, each glass receiver surface 1324, 1326 iscoated with an idealized material that transmits approximately 98% ofthe incident light (independent of incidence angle) and specularlyreflects the remaining 2%. The transmittance of the glass envelope tubeis approximately 96% and is consistent with figures from literature forthe state-of-the-art receiver. The absorber tube surface 1318 is modeledsuch that 95% of incident light is absorbed and the remaining 5% isscattered according to a Lambertian model. Because we are currentlyfocused on receiver optical performance numbers, the collector ismodeled as being 100% reflective and free of dirt.

The Zemax software tool rigorously accounts for energy in the simulatedsystem and as a result, can be used to evaluate critical parameters suchas the optical efficiency. For example, a power of 1 W was assigned tothe solar source used in the simulated system shown in FIG. 13. Tomeasure the collected light, a series of cylindrical detectors were usedto determine the fraction of incident light that ultimately reached thecentral absorber surface. A first detector placed across the entirewidth of the collector aperture (1304) determined the total amount ofradiation incident on the collector. This detector measured 0.926 W (fora 100,000 ray Monte Carlo simulation)—this value is consistent with thesource size, which was selected to extend somewhat beyond the collector.A second cylindrical detector was positioned just outside the receiverglass envelope (1310)—this source also measured 0.926 W, indicating thatin the simulation the all light from the parabolic collector strikes thereceiver, despite the angular extent of the sun. A third cylindricaldetector, positioned immediately inside the glass envelope, determinedthe amount of radiation lost due to reflection at the glass interfaces.This detector measured 0.888 W. This 4% loss relative to the previousdetector is consistent with stated values in the literature. The finaldetector recorded the amount of radiation that was absorbed by thecentral tube. This detector measured 0.844 W, corresponding to a 95%which is consistent with the value of the tubes' absorptivity. Theoverall efficiency of the receiver is 0.844 W/0.926 W=91%. With theinclusion of 2% for shading and optical errors, this value closelymatches that stated in the literature.

FIG. 14 demonstrates the ability of Zemax to incorporate collectorerrors into the model. In this method a prescribed amount of scatteringis implemented at the collector surface allowing for uncertainty in theray paths to be introduced in a controllable manner. This Gaussian errordistribution combines with the angular extent of the sun to determinethe focal volume. Another method of introducing collector error would beto model specific mirror deformations and calculate the resulting focusdegradations directly. FIG. 14 shows the percentage of radiation fromthe collector that strikes a 7-cm-diameter receiving tube (1402, 1404,1406) for a 10 mrad (FIG. 14A), 5 mrad (FIG. 14B) and 0 mrad (FIG. 14C)collector errors. In FIG. 14C the rays exhibits a finite focal volume(i.e., the rays do not intersect at a single point but in a volume sizedsuch that all rays strike the receiver) due to the sun only. When thecollector error is increased to 5 mrad (FIG. 14B), 0.6% of the lightrays (e.g., light ray 1408) now miss the circular tube, while for acollector error of 10 mrad (FIG. 14A), the mirror errors have expandedthe focal volume such that a significant fraction of energy (3.8%)misses a 7-cm-diameter receiver tube (e.g., light ray 1110).

Current collector designs have a typical error of 5.4 mrad resulting ina simulated collector optical efficiency of approximately 99% for atypical state-of-the-art vacuum tube system. When combined with the 91%simulated receiver efficiency for a perfect mirror, the total opticalefficiency of the simulated collector/receiver system is 90% excludingshading of the collector by to the metal bellows. With inclusion ofbellows shading, the result is comparable to the 89% efficiency of theSOA system found in the literature. The very close correspondence ofthese numbers serves to validate our optical modeling of the radiationconcentration, transmittance, and absorption processes of a SOAcollection-receiver system.

Investigations into reducing collector optical errors set a total systemerror target of 2.5 mrad. These reductions are regarded as beingattainable given the ˜1.5 mrad errors achieved by dish-Stirlingprototypes. Typical system errors are currently 5.4 mrad. The targetvalue was based on reducing, the slope, alignment, tracking and windloading errors to 1, 0.5, 1, and 2 respectively, giving an overallsystem error of 2.5 mrad. Industry has made progress towards these goalswith Gossamer Space Frames recently reporting that their slope error isclose to 1.1 mrad. In our analysis of an optimized system for the planarreceiver a more conservative target of 3 mrad is used.

For a collector error of 5.4 mrad, 99% of incident light hits a 7 cmdiameter absorber tube. Therefore, the majority of optical losses in thestate-of-the-art design do not occur from rays missing the absorber. Inthe model, optical losses are primarily due to the fact that thetransmittance of the glass envelope and absorptivity of the absorbingsurface are not 100%. In addition, optical performance will be reducedby bellows shading (˜1-2%), and from non-ideal mirror reflectivity andcleanliness. These losses will occur in all systems that involve amirror, glass tube and absorber tube. Various embodiments of thisinvention may have an optical advantage relative to the state-of-the-artdesign through the elimination of a glass cover and by eliminatingbellows shading.

FIG. 15 depicts the optical efficiency of embodiments of the inventionthat varies the focal length and size of receiver tube as means ofreducing thermal losses without a loss in optical efficiency. In thisfigure, the optical efficiency assumes that the absorbing tube has 100%absorptivity as opposed to 95% (as in FIG. 13), to focus on thegeometric factors that affect optical efficiency. In these simulations,the geometry of the receiver envelope is kept constant and consistentwith the Schott PTR70 receiver. The inner and outer glass surfaces havea transmittance of 98% and the collector aperture is fixed at 5 m. FIG.15 shows the effects of modifying the absorber diameter and focal lengthon the optical efficiency of the system for collector errors of 3.0 mrad(FIG. 15A) and 5.4 mrad (FIG. 15B). FIG. 15 demonstrates that the SchottPTR70 is already well-optimized for optical efficiency. Furtherincreases to the absorber diameter do not significantly increase theoptical efficiency but will act to reduce thermal efficiency. Likewise,for a collector error of 5.4 mrad, the optimal performance deterioratesrapidly as the absorber diameter decreases from 6 cm to 3 cm. Theanalysis demonstrates that a focal length of 1.5 m (rim angle of 80°) asused in the Schott PTR70 is optimal for a cylindrical receiver. Atlarger focal lengths, more of the incident light misses the receiver todue to the increased spreading of the light as a result of collectorerrors and the angular distribution of the sun.

FIG. 16 depicts a theoretical geometric analysis of the focal-spot sizeof a planar receiver and a cylindrical receiver for a linear trough. Theacceptance angle (typically the sum in quadrature of the solar angularextent and the collector errors) is projected from the collector to thereceiver in order to determine a focus extent. In this analysis, thecalculated focal size is chosen so that approximately 95% of thereflected light from the collector strikes the receiver. The focal sizeis affected by the collector error and the angular distribution of thesun. In this calculation the collector error is 5.4 mrad and the angulardistribution of the sun is 4.7 mrad. The concentration ratio is definedas the collector aperture diameter divided by the absorber width for aplanar receiver and collector aperture diameter divided by absorberdiameter for a cylindrical receiver.

For a given collector error, the theoretical planar receiver has a lowermaximum concentration ratio than a cylindrical receiver. The higherconcentration ratio of the cylindrical receiver does not necessarilycorrespond to an improved thermal performance since the exposed area ofthe cylindrical absorber is the circumference and not the diameter. Therim angle that produces the maximum concentration ratio is smaller forthe planar receiver than the cylindrical receiver. For a planar receiverthe optimum rim angle is 55-65° (1602), as compared to 90° (1604) forthe cylindrical receiver. This analysis does not include the effects ofshadowing of collector by the planar receiver or the reduction inoptical efficiency from a glass envelope or covering. The maximumconcentration ratio of the planar receiver can be improved through theuse of secondary non-imaging optics such as a compound parabolicconcentrator, a lens or other non-imaging optical devices.

FIG. 17 shows the optical efficiency of a planar receiver calculatedusing Zemax for a range of focal lengths and absorber widths. In thisanalysis the collector diameter was fixed at 5 m and the collector errorset to be 3 mrad. The optical efficiencies of a receiver without a glasscover included at the base of the cavity are shown in FIG. 17A and theoptical efficiencies of a receiver with a glass cover are shown in FIG.17B. When glass is used in the receiver design the maximum opticalefficiency is approximately 1% lower than the state-of-the-art design.The improvement in optical efficiency is approximately 3-4% in theembodiment where a glass covering is not included which is consistentwith the transmittance percentage of the glass. The optimal focal length(rim angle) for a planar design is longer (smaller) than for thestate-of-the-art which is consistent with theoretical analyses shown inFIG. 16.

FIG. 18 shows the geometry for the compound parabolic concentrator(CPC), which is a non-imaging optical device used to concentrate lightincident on a planar entrance aperture. A CPC is used to concentratelight incident on a planar entrance aperture of length d₂ (1802), withinan acceptance angle 1804, to a smaller exit aperture of length d₁(1806). The exit aperture 1806 may be occluded by, e.g., the absorbingsurface of a heat-absorbing element. The light at the exit aperture willnecessarily occupy a larger propagation angle, as dictated by étendueconsiderations. The CPC consists of two parabolic mirrors (1808, 1810),which form the entrance aperture (1802) and receiver opening (1806). Thetruncated portions of parabolic mirrors 1808 and 1810 are shown bysections 1820 and 1582 respectively. Provided that light enters the CPCwithin the acceptance angle (1804), it will exit the receiver openingeither directly or after one reflection from the CPC walls. The focalpoint of parabola 1808 is on parabola 1810 and is located at 1812. Thefocal point of parabola 1810 is on parabola 1808 and is located at 1814.If the incoming light is parallel to the parabola axis (i.e., first axis1816 for parabola 1808, second axis 1818 for parabola 1810), then thelight will be focused on the relevant focal point. If the light entersat an angle of less than half of the acceptance angle the light will befocused onto the receiver opening (1806). If the light enters an anglegreater than half of the acceptance angle then the light may bereflected back out of the aperture opening.

Assuming the exit aperture (dictated by the desired receiver size) andthe acceptance angle (θ), which will depend on the collector rim angle,are defined, the entrance aperture is given by

$d_{2} = \frac{d_{1}}{\sin \left( {\theta/2} \right)}$

and the CPC length (L) is given by

$L = {\frac{d_{1}\left\lbrack {1 + {\sin \left( {\theta/2} \right)}} \right\rbrack}{2{\tan \left( {\theta/2} \right)}{\sin \left( {\theta/2} \right)}}.}$

These equations are sufficient to fully specify the CPC. Theconcentrating power of the CPC increases as the acceptance angle (andrim angle decreases). However as the rim angle decreases the length ofthe CPC increases. In addition, the increased width of the CPC at lowerrim angles increases the shadowing of the collector by the receiverreducing optical efficiency. The length and width of the CPC becomesprohibitive to the use of lower rim angles (longer focal lengths).

FIG. 19A and FIG. 19B compare the optical efficiencies of a planarreceiver with a 100% reflective CPC (FIG. 19A1, FIG. 19B1) and 95%reflective CPC (FIG. 19A2, FIG. 19B2) for a range of absorber sizes andfocal lengths.

Reference is now made to FIG. 19A, which pertains to a receiver withouta glass aperture cover. When the CPC is 100% reflective (FIG. 19A1) theoptical efficiency is improved for smaller absorber widths compared tothe embodiment without a CPC (FIG. 18). This improvement corresponds toan increase in the concentration ratio for a given optical efficiency.However, the CPC increases the shadowing of the collector as evidencedby the decrease in optical efficiency for larger absorber size. Sincethe concentration ratio (and therefore width) of the CPC increases atlarger focal lengths (smaller rim angles), this effect is more apparentwhen the focal length is longer.

When the reflectivity of the CPC is reduced to 95%, the opticalefficiency and concentration ratios are reduced. At a focal length of2.5 m and an absorber width of 7 cm the optical efficiency is decreasedapproximately 2-3% compared to the 100% reflective CPC. The optimumfocal length for a given absorber size is decreased when the CPCreflectivity is reduced from 100% to 95%. The optical efficiency of theplanar receiver with 95% reflective CPC is lower than the planarreceiver without a CPC over much of the range of absorber widths andfocal lengths. However, the optical advantage of the CPC is stillapparent at lower absorber widths and longer focal lengths.

Reference is now made to FIG. 19B, which pertains to a receiver with aglass aperture cover. The addition of glass to the bottom of thereceiver decreases the optical performance universally by approximately4%. A glass covering may be included to reduce thermal losses at thebase of the receiver due to forced convection from the wind. When glassis included, the peak optical efficiency of the receiver with a 95%reflective CPC (FIG. 19B2) is 3% lower than the Schott PTR70. Withoutglass, the optical performance of this receiver exceeds that of theSchott PTR70 by 1%.

FIG. 20A and FIG. 20B display illustrative plots relevant to theinvention. Specifically, the illustrative plots may be employed in amethod whereby the optimum receiver size (e.g., width of theheat-absorbing surface 418 in FIG. 4) is determined for a giventemperature of operation of the heat-transfer-fluid (e.g., fluid 406 inFIG. 4) by maximizing the performance of the receiver at that operatingtemperature.

FIG. 20A displays a plot of energy gained by the receiver versusreceiver size. The energy gained by the receiver is plotted on they-axis and is based on optical and thermal analyses. The x- and y-axesare not given numeric values, as the general shape of the three curvesare universal and demonstrate the procedure for determining the optimalpoint.

The optical energy intercepted 2002 is calculated from ray-tracingsoftware (Zemax) and based on parameters such as optical error, receiversize, mirror size, and mirror shape. The energy gained is calculatedfrom the power per unit length (e.g., Watts/meter) intercepted by thereceiver in the ray-tracing software. The characteristic shape of theoptical energy intercepted by the receiver as a function of receiversize is displayed in FIG. 20A. where optical energy interceptedincreases rapidly with receiver size until all the light is intercepted(just about the point 1708, i.e., optimum receiver size). For a givensize mirror (e.g., 5 meters), optical error (e.g., 5 mrad), nearly all(e.g., over 99%) of the optical radiation is intercepted for a planar agiven receiver size (e.g., 10 cm). After that, the optical energyintercepted stays almost constant, as indicated by the long nearly flatpart of the curve, with a slight decrease due to shading caused by thelarger receiver size.

The thermal energy lost from the receiver 2004 is calculated from CFDsoftware (ANSYS Fluent) and based on parameters such as receiver size,heat transfer fluid temperature, absorber emissivity, and heat transferproperties. The energy lost is calculated from the power (heat) per unitlength (e.g., Watts/meter) transferred to the environment in the CFDsoftware. The characteristic shape of the thermal energy loss by thereceiver as a function of receiver size is displayed in FIG. 20A, wherethermal energy loss increases nearly linearly with receiver size(exposed absorber size).

The total energy gained by the receiver 2006 is calculated as the sum ofthe optical energy gained 2002 and thermal energy lost 2004. To maximizethe total energy gained by receiver, the receiver size would be chosenas the peak 2008 of the curve 2006. This optimum receiver size 1708 isdependent on operating parameters such as operating heat transfer fluidtemperature, absorber emissivity, receiver heat transfer properties,optical error, mirror size, and mirror shape. In this manner, optimumreceiver size can be determined, improving overall performance of asolar trough-based concentrating solar plant.

FIG. 20B displays three plots of energy gained by the receiver versusreceiver size for different heat transfer fluid temperatures. The energygained by the receiver is plotted on the y-axis and is based on the sameoptical and thermal analyses described in FIG. 20A. The x and y axes arenot given numeric values, as the general shape of the three curves areuniversal and demonstrate the procedure for determining the optimalpoint.

The y axis displays the total energy gained by the receiver and, as inFIG. 20A, is calculated as the sum of the optical energy gained andthermal energy lost. Three curves are shown for different heat transferfluid temperatures within the receiver: curve 2052 displays the totalenergy gained by the receiver for a heat transfer fluid temperature of400° C., curve 2054 displays the total energy gained by the receiver fora heat transfer fluid temperature of 550° C., and curve 2056 displaysthe total energy gained by the receiver for a heat transfer fluidtemperature of 650° C. Other operating parameters such as absorbercoating, receiver heat transfer properties, optical error, mirror size,and mirror shape are kept the same for the three analyses. The optimumreceiver size can be determined for each operating temperature: point2062 represents the optimum receiver size for 400° C., point 2064represents the optimum receiver size for 550° C., and point 2066represents the optimum receiver size for 650° C. Receiver size can bevaried within a solar trough-based concentrating solar plant based ontemperature to improve overall plant performance. Likewise, if formanufacturability, cost, or other reasons, a single receiver size ispreferred for a solar trough-based concentrating solar plan, a singleoptimum receiver size can be determined averaging over temperatures andparameters, improving overall performance of a solar trough-basedconcentrating solar plant.

FIG. 21 cross-sectionally illustrates an illustrative embodiment (FIG.21A) of aspects of the invention and a comparison (FIG. 21B) of thesimulated heat loss of the embodiment 2100 with the measured andextrapolated heat loss of a state-of-the-art (SOA) receiver constructedaccording to the prior art (i.e., the Schott PTR70). The receiver 2100comprises a tubular envelope 2102, a tubular heat-absorbing element 2104through which a heat-transfer fluid 2106 can flow, a heat-absorbingcoating 2108 upon the outer surface of the heat-absorbing element 2104,an outer insulating jacket 2110 that covers a part of the outer surfaceof the envelope 2102, and an aperture 2112, not covered by insulation2110, through which light can pass through the envelope 2102. Theannulus or space 2114 between the heat-absorbing element 2104 and theenvelope 2102 is vacuum-filled. The receiver 2100 may or may not includea reflective coating 2116 on the inner surface of the envelope 2102except over the aperture 2112.

FIG. 21B compares measured and extrapolated heat-loss performance of theSOA receiver to the simulated performance of the receiver 2100, wherethe latter is simulated both with and without a reflective coating 2116.As the plot makes clear, the receiver 2100 has lower heat loss at alltemperatures than the SOA receiver, with relatively greater improvementat higher temperatures. The plot also shows that the presence or absenceof a reflective coating 2116 on the inner surface of the envelope 2102(“Coated Cavity” points vs. “Uncoated Cavity” points) makes relativelylittle difference to heat loss. Compared to the structurally similarnon-cavity SOA receiver, the addition of insulation to the cavityreceiver 2100 makes most of the difference.

NREL data (i.e., Burkholder and Kutscher 2008, described above for FIG.5A) for the Schott PTR70 did not exceed 500° C., and the cermet absorbercoating of the Schott PTR70 cannot endure prolonged exposure totemperatures significantly above 500° C.; therefore, the heat-lossfigures for “SOA” receiver in FIG. 21 for temperatures above 500° C. arebased on extrapolation of the NREL data, making the assumption that thecermet absorber coating of the SOA receiver remains stable at such hightemperatures. In other words, actual SOA performance would be worse at550° C. and 650° C. than is depicted in FIG. 21B. Figures for thereceiver 2100 at all temperatures are based on the ANSYS Fluentsimulation tool described above for FIG. 5A and FIG. 6. This simulationassumes in the receiver 2100 an absorbent coating with the sametemperature-emissivity curve as the coating in the Schott PTR70receiver—as opposed to a more advanced coating—but with the additionalproperty of being stable up to at least 650° C. The external insulation2110 was modeled based on the properties of Microtherm Super G typeinsulation, which has a thermal conductivity of 0.02 W/m*K at 300° C.For Microtherm Super G insulation, 2 cm of insulation has a thermalresistance of 1 K*m²/W.

FIG. 22A1 and FIG. 22A2 are cross-sectional plots of simulated contoursof constant temperature within (a) (FIG. 22A1) an illustrativestate-of-the-art receiver 2200 similar to the Schott PTR70 receiver andconstructed according to the prior art, but with air rather than vacuumin the annulus 2202, and also within (b) (FIG. 22A2) an illustrativereceiver 2204 similar to receiver 2100 in FIG. 21 and also containingair in the annulus. Receiver 2204 incorporates aspects of the invention.Both receivers 2200, 2204 include a transparent tubular envelope 2208,2210 and a tubular heat-absorbing element 2212, 2214. The heat-absorbingelements 2212, 2214 contain a heat-transfer fluid 2216, 2218. Theinsulated receiver 2204 includes an insulating jacket 2220 partlycovering the outer surface of the envelope 2210; the envelope 2210includes a reflective coating 2222 on its inner surface. Both receivers2200, 2204 are simulated at an operating at temperature of 400° C.(fixed temperature of the absorber coating). An absorber coatingcomposition for receivers 2200, 2204 and an insulation composition forthe cavity receiver 2204 are both assumed, for this analysis, as for theanalysis providing the results plotted in the lower portion of FIG. 21.

FIG. 22A3 and FIG. 22A4 are plots of velocity vectors for convecting airfor the receivers 2200, 2204, again at an operating temperature of 400°C.

FIG. 22A shows that conviction is more vigorous in an air-filled,non-insulated receiver 2200 than in an air-filled, insulated receiver2204.

FIG. 22B1 is a cross-sectional illustration defining energy loss fluxesfor the air-filled, insulated receiver 2204 from FIG. 22A4.“Qinsulation” (arrow 2224) is the heat flux through theinsulation-covered portion of the envelope 2210. “Q_Radiation” (arrow2226) is the radiation flux through the non-insulation-covered portionof the envelope 2210. “Q_Cond_Conv” (arrow 2228) is the heat flux viaconduction through the non-insulation-covered portion of the envelope2210. A Q-Radiation flux and Q_Cond_Conv flux may be similarly definedfor the uninsulated receiver 2200 in FIG. 22A1, also termed herein the“bare receiver”; there is no Q_insulation flux for the bare receiver2200.

FIG. 22B2 is a chart of the three fluxes for both the bare receiver andthe insulated receiver for operating temperatures of 400° C., 550° C.and 650° C. The chart shows clearly that the Q-Radiation flux andQ_Cond_Conv fluxes are significantly smaller for the insulated receiver2204 than for the bare receiver 2200. The strong loss reductions seenwith adding insulation to the receiver exterior suggest that gas-filledreceivers are feasible in terms of efficiency when combined with aninsulated covering.

FIG. 23A is a cross-sectional plot of simulated contours of constanttemperature in an illustrative receiver 2300 incorporating aspects ofthe invention. Receiver 2300 resembles the bare receiver 2200 in FIG.22A1, but with the addition of six barriers (e.g., barrier 2302)connecting the envelope 2304 to the heat-absorbing element 2306 in agas-proof manner and so dividing the air-filled annulus (space betweenthe envelope 2304 and heat-absorbing element 2306) into six cells (e.g.,cell 2308) running the length of the receiver 2300. Addition of dividersto segment the annulus into cells to reduce convection might beattempted as an alternative or supplement to the addition of aninsulating jacket over a bare receiver, with the goal of reducing heatlosses from the receiver; however, as FIG. 23B shows, in simulations,that the addition of two or six dividers to an otherwise identical barereceiver actual increases convection heat loss from the receiver. Itappears the divider strategy fails because it does not alter surfacetemperatures or cylinder diameters, which are the driving parameters ofannular-flow heat transfer. However, the plot of lines of constanttemperature in the upper portion of FIG. 23A does reveal one benefit ofdividers: isolating a section of air below the hot absorber creates astratified (horizontally layered) zone with minimal convection currents.Stratification tends to retain heat energy hear the heat-absorbingelement (top of the cavity) rather than transporting it to the envelope(bottom of the cavity), and thus is advantageous in reducing thermallosses from the receiver.

FIG. 24A1 cross-sectionally illustrates an illustrative receiver 2400incorporating aspects of the invention. Receiver 2400 resembles the barereceiver 2200 in FIG. 22A1, but with the addition of an insulatingjacket 2402 partly filling the annulus between the heat-absorbingelement 2404 and the transparent tubular envelope 2406. The portion ofthe annulus not filled by the insulating jacket 2402 constitutes anair-filled optical cavity 2408 that light may enter by passing through aportion 2410 of the envelope 2106; this portion 2010 of the envelopeconstitutes an optical aperture.

FIG. 24A2 is a cross-sectional plot of simulated contours of constanttemperature within the insulating jacket 2402 and air-filled opticalcavity 2408 of receiver 2400 at an operating temperature of 400° C. Anabsorber coating composition for receivers the heat-absorbing element2104 and an insulation composition for the insulating jacket 2402 areboth assumed, for this analysis, as for the analysis providing theresults plotted in FIG. 24A2.

In FIG. 24A2, the lines of constant temperature in the optical cavity2408 reveal a stratified (horizontally layered) zone with minimalconvection currents. As noted above in the description of FIG. 23A,stratification tends to advantageously reduce thermal losses.

FIG. 24B1 is a cross-sectional illustration defining energy loss fluxesfor the receiver 2400 from FIG. 24A1, also herein termed the “wedge”receiver. “Qinsulation” (arrow 2410) is the heat flux through theportion of the envelope 2406 overlying the insulating jacket 2402.“Q_Radiation” (arrow 2412) is the radiation flux through the portion ofthe envelope 2406 covering the cavity 2408. “Q_Cond_Conv” (arrow 2414)is the heat flux via conduction through the portion of the envelope 2106covering the cavity 2408.

FIG. 24B2 is a chart of the three fluxes for both the bare receiver 2200of FIG. 22A1, the externally insulated receiver 2204 of FIG. 22A2, andthe internally insulated receiver 2400 of FIG. 24A1, for operatingtemperatures of 400° C., 550° C. and 650° C. The chart shows clearlythat the Q-Radiation flux and Q_Cond_Conv fluxes are significantlysmaller for the internally insulated receiver 2400 than for either thebare receiver 2200 or the externally insulated receiver 2204. Bydirectly covering the majority of the surface of the heat-absorbingelement 2404, reducing the size of the air pocket, and inducingstratification, the wedge receiver 2400 substantially reduces bothradiative and convective losses.

FIG. 25A demonstrates the effect of receiver tilt angle on the thermalperformance of the receiver. As the position of the sun moves throughoutthe day, the receiver/collector system must tilt in order to maximizethe incident solar radiation. If the sun is directly overhead then thetilt angle is defined as being 0° and the receiver cavity pointsvertically down. If the sun is at the horizon then the tilt angle is90°.

FIG. 25A shows the effect of the receiver tilt on heat loss for thecurrent receiver design at an operating temperature of 650° C. Thisembodiment consists of an absorber with a width of 5 cm, a compoundparabolic concentrator, a focal length of 2.5 m, 3.5 cm of Microtherminsulation, and a glass cover. The heat loss at a tilt angle of 80° isapproximately 17% higher than at 0° (2502). The increase in heat loss asthe receiver tilts from 0° to 80° results in a decrease of totalreceiver efficiency from 78.5% to 77.3%. At a tilt angle of 40° the heatloss increases by 7% and the total receiver efficiency is decreased to78.0%. The radiation heat loss remains relatively constant as the tiltangle changes (2404). This initial analysis demonstrates that theenhanced heat loss at larger receiver tilt angles is not prohibitive tothe high total efficiency of the receiver. Upcoming analysis willpredict an average efficiency based on the angular position of thecollector over the course of a day.

FIG. 25B shows velocity vectors for three tilt angles: 0° (2306), 40°(2308) and 80° (2510) for the current leading receiver design at 650° C.FIG. 25B demonstrates that as the tilt angle increases from 0° to 80°,the stratified layer of air in the receiver (2512) breaks down and aone-cell convection pattern forms (2514). The magnitude of theconvective velocity increases as the receiver is tilted from 0° to 80°.The enhanced convection facilitates the transfer of energy from theabsorber (2516) to the glass covering (2518) where it may be lostthrough the glass by conduction.

FIG. 25C shows contours of constant temperature for three tilt angles:0° (2520), 40° (2522), and 80° (2524) for the current leading receiverdesign at 650° C. As the tilt angle increases the temperature of the airin the vicinity of the glass aperture is increased due to the enhancedconvective transfer of energy from the absorber to the glass covering.The increased temperature of the fluid at the glass covering will resultin larger heat transfer across the glass window due to conduction. Inother embodiments of the invention, the material used for the cover ischanged in order to reduce thermal losses while maintaining a highoptical efficiency. Optimization of the position of the covering andlength of the receiver legs is possible to reduce heat loss due to atilted receiver.

FIG. 26 displays plots of constant temperature contours for twoillustrative embodiments of the invention. Specifically, FIG. 26cross-sectionally depicts results of a CFD analysis of an illustrativenovel receiver that incorporates aspects of the invention, including aselective aperture cover as described in FIG. 27.

The CFD analysis displayed is for an illustrative advanced cavityreceiver in two states, i.e., (1) without a selective insulatingaperture cover (receiver 2602) and (2) with a selective insulatingaperture cover 2604 (receiver 2604). Receivers 2602, 2604 comprise aninsulating jacket 2606 partially surrounding a heat-absorbing element2408 above a gas-filled (e.g., atmospheric air) cavity, and an aperture2612 with a transparent aperture cover 2612 through which light isconcentrated by mirror collector (not shown) onto heat-absorbing element2608. Receivers 2602, 2604 are approximately uniform in cross-sectionalong its entire length, apart from mounting and other hardware (notshown) at each end. Receivers 2602 and 2604 are identical except thatreceiver 2604 has been closed by a selective insulating aperture cover2614.

Displayed in FIG. 26 are dashed lines of constant temperature based on aCFD analysis where the heat-absorbing element 2608 is fixed at operatingtemperature 650° C. Contour lines are not shown in the selectiveinsulating cover 2614. Heat loss by the receiver 2604 is reduced by theaddition of the cover as compared with heat loss by receiver 2602. Forthe case shown, with operating temperature of 650° C., the heat lossdecreases from 374 W/m in the receiver 2602 without the selectiveinsulating cover, to 159 W/m for the receiver 2604 with the selectiveinsulating cover 2614. This reduction by over half in heat loss byaddition of selective insulating cover 2614, indicates that performanceof a trough-based concentrating solar plant may be substantiallyimproved by reducing thermal losses during idle times. CFD analyses forother operating temperatures were completed (not shown), showing thatwhen operating at 550° C., the heat loss decreases from 256 W/m in thereceiver 2602 without the selective insulating cover, to 147 W/m for thereceiver 2604 with the selective insulating cover 2614. Additionally,when operating at 400° C., the heat loss decreases from 144 W/m in thereceiver 2402 without the selective insulating cover, to 112 W/m for thereceiver 2604 with the selective insulating cover 2614.

FIG. 27 displays another illustrative embodiment of the invention.Specifically, FIG. 27 cross-sectionally depicts features of anillustrative novel receiver that incorporates aspects of the invention.

The illustrative advanced cavity receiver with selective aperture cover2700 comprises a tubular shell 2702, a heat-absorbing element 2704through which a heat-transfer fluid 2706 can flow, an aperture 2712,(with optional transparent aperture cover (not shown)), an insulatingjacket 2716 partially surrounding the heat-absorbing element 2704, aplanar (flat) absorbing surface 2718 coated with an absorbent coatingand exposed to light entering the aperture 412. The receiver 400 isapproximately uniform in cross-section along its entire length, apartfrom mounting and other hardware (not shown) at each end.

Additionally, the receiver may include a moveable aperture cover 2720.The cover 2720 may, for example, be a transparent material such asglass, a structural material such as plastic or metal, or an insulatingmaterial such as opaque insulation. The cover 2720 may be rotated orother moved to cover the receiver aperture opening 2712. In thisillustrative example, the cover 2720 may be mounted to a circular track2730 at both the lengthwise (coming out of the page) front and back endof the receiver. A motor (not shown) or other rotary device (e.g.,manual crank) may be used to rotate the aperture cover 2720 from a fullopen position as shown in 2700 to a full closed position as shown in2770. A mechanical stop (e.g., a fixed piece of metal at the lengthwisefront and back end of the receiver) 2734 may be used to assure properseating and location of the aperture cover when rotated full open 2700or full closed 2770. Arrow 2732 indicates the direction of rotation toclose the aperture cover 2720.

Performance (e.g., overall efficiency) may be improved during differentoperating conditions by the selective covering of the aperture opening2712 with the cover 2720. For example, during windy conditions, a glassaperture cover may be rotated into place to reduce convective thermallosses. The glass will result in reduced optical efficiency, but inconditions of high wind, this loss may be more than compensated by gainsin thermal efficiency. In another embodiment, additional insulation 2722may be added to the cover 2720. This insulating covering 2720 2722 maybe rotated into place to further reduce thermal losses during night timeand other times of little or no sunlight to reduce thermal losses.

This illustrative cavity receiver 2700 can be the same as shown in FIG.7 with the addition of the selective insulated aperture cover. In otherembodiments, other receiver geometries (e.g., that of receiver 400 inFIG. 4) may include the selective insulated aperture cover.

2750 shows the same illustrative cavity receiver 2700, but with theaperture cover 2720 and insulating covering 2722 rotated halfway towardsclosing against stop 2734 on track 2730. Arrow 2752 indicates thedirection of rotation to close the aperture cover 2720.

2770 shows the same illustrative cavity receiver 2700, but with theaperture cover 2720 and insulating covering 2722 rotated completelytowards closing against stop 2734 on track 2730.

In other embodiments, not shown, the selective aperture cover 2720, 2722could be internal to the stationary insulation 2716.

FIG. 28 and FIG. 29 show the effect of forced convection (wind) on thethermal performance of an embodiment of the receiver with no glass coverat the cavity aperture. In this embodiment, the cavity and absorbingsurface are open to the environment. FIG. 28A, FIG. 28B, FIG. 28C andFIG. 28D show the convective velocity in and around the open receiverfor a wind speed of 2.2 m/s incoming at a direction of 0°, 15°downwards, 15° upwards and 30° upwards respectively. The simulationswere performed for an illustrative receiver with a compound parabolicconcentrator, a 2.5 m focal length, 2 cm external Microtherm insulation,7 cm absorber width and an operating temperature of 550° C. For thereceiver which has a glass cover and a sealed air-filled cavity atatmospheric pressure, the heat loss is 342 W/m. In an embodiment of theinvention, the evacuation of the cavity results in a reduction of theheat loss to 293 W/m. Embodiments of the invention where a portion ofthe cavity is at vacuum will result in improvements to the thermalefficiency and may improve absorber coating stability. When the glasscover is removed and an external wind speed applied at 2.2 m/s the heatloss is increased between 11% and 80% depending upon the direction ofthe incoming wind.

The removal of the glass covering increases the optical performance byapproximately 4% (the transmittance percentage of the glass). In orderto achieve gains in total efficiency when the glass cover is removed inthis example the increase in heat loss must be less than 58% (absoluteheat loss less than ˜530 W/m). Hence, these initial results suggest thatthe removal of the glass cover may improve the total efficiency of thereceiver over a range of wind velocities.

These initial simulations were performed for a receiver tilt angle of0°. For an embodiment of the receiver with no glass covering the tiltangle will further affect the heat loss of the receiver due to naturaland forced convection. Embodiments of the invention include a receiverwith no glass cover that is optimized to reduce heat loss for a varietyof tilt angles.

FIG. 30 cross-sectionally illustrates two illustrative advanced cavityreceivers, 3000 and 3002, embodying aspects of the invention. Receiver3000 (FIG. 30A) resembles receiver 400 in FIG. 4, but includes glasssections (3004 and 3006) at the cavity entrance that form a partialglass cover rather than a full glass cover. Receiver 3002 (FIG. 30B)resembles receiver 700 in FIG. 7 but includes glass sections (3008 and3010) at the cavity entrance that form a partial glass cover rather thana full glass cover. The middle section (3012 in receiver 3000 and 3014in receiver 3002) of the cavity aperture is left open.

The percentage of the cavity opening that is covered by glass is chosento maximize the total receiver efficiency. Increasing the percentage ofthe cavity opening that is covered by glass will increase the thermalperformance at a cost to the optical performance. The optimum percentagewill depend upon the profile of the irradiant flux on the cavity openingand the extent to which the width of the partial glass cover affects thethermal performance.

In one embodiment of the invention, the percentage of glass covering thecavity opening may be variable and can be altered depending upon thecurrent weather conditions.

FIG. 31 displays another illustrative embodiment of the invention.Specifically, FIG. 31 shows solar power system 3100 which includes analternative embodiment of a solar field 3102 to replace or supplementsolar field 102 in FIG. 1 for a trough-based concentrating solar powersystem (100 in FIG. 1). The solar field is controlled to track the sunvia controller 3180.

The solar field 3102 comprises N rows (e.g., row 3104, row 3106),depicted in FIG. 31 as viewed from above, where each row comprises Mreceivers (e.g., receivers 3108 a, 3108 b; receivers 3110 a, 3110 b,receivers 3112 a, 3112 b; etc.) upon which solar radiation is focused byparabolic collectors (not depicted). The plurality of the N rows isindicated in FIG. 1 by horizontal ellipses (e.g., ellipsis 3116); theplurality of M receivers in each of the N rows is indicated in FIG. 1 byvertical ellipses (e.g., ellipses 3114 a, 3114 b). The number ofreceivers K in the solar field of the illustrative system 100 istherefore K=NM. In other embodiments, the number of receivers may varyfrom row to row.

In one state of operation, a heat-transfer fluid (e.g., molten salt,refractory oil, gas, pressurized water or steam) is admitted to an entrymanifold 3118 at a first temperature, T₁ (e.g., 200° C.). (In FIG. 31,any piping through which the heat-transfer fluid may flow in some stateof operation of the system 3100 is denoted by a thick dark line.) Theheat-transfer fluid is distributed by the entry manifold to the N rowsand, in each row, passes sequentially through the M receivers comprisedby that row. In general, an approximately equal amount of solarradiation Q_(c) is gathered by the collector associated with eachreceiver, and absorbed by the fluid in each receiver with an efficiencyη_(r). Thus, an approximately equal quantity of solar energyQ_(r)=Q_(c)η_(r) is added to the heat-transfer fluid during its passagethrough each receiver. Each incremental set of the M receivers will thusraise the thermal energy and thus, for non-phase change heat transferfluids, raise the temperature of the heat-transfer fluid. The first setof receivers (3108 a and 3108 b), which consists of a given length ofreceivers (e.g., 40 meters), will raise the heat transfer fluid from T₁to a second elevated temperature T₂ (e.g., 250° C.) higher than the T₁.For simplicity, the average temperature T₁₂, (e.g., 225° C.) of thefirst set of receivers 3108 may be considered the average of T₁ and T₂(T₁₂=(T₁+T₂)/2). The second set of receivers (3110 a and 3110 b), whichconsists of a second given length of receivers (e.g., another 40 metersor some other length not necessarily equal to the first length), willraise the heat transfer fluid from T₂ to a third elevated temperature T₃(e.g., 300° C.). The second set of receivers 3110 operates at averagetemperature T₂₃ (e.g., 275° C.) higher than the average operatingtemperature, T₁₂, of the first set of receivers 3108. In the samemanner, the third set of receivers 3112 has a given length, inlettemperature T₃, elevated outlet temperature T₄ (e.g., 350° C.), andaverage operating temperature T₃₄ (e.g., 325° C.). Additional sets ofreceivers 3114 may raise the temperature further to the outlet operatingtemperature where it is admitted to an output manifold 3120 at a finaltemperature, T_(f) (e.g., 550° C.).

Detail 3130 shows an exemplary cross-section of the receiver in thefirst receiver sections 3108 a and 3108 b which typically operates atelevated operating temperature T₁₂. The receiver is similar to thereceiver 700 in FIG. 7. The receiver 3130 has an exposed opening ofwidth 3140. In various other embodiments, receivers may be used thatcomprise planar transparent aperture covers, lenticular aperture covers,partial aperture covers, selective insulated aperture covers, compoundparabolic concentration, and other aspects of the invention.

Detail 3132 shows an exemplary cross-section of the receiver in thesecond receiver sections 3110 a and 3110 b which typically operates atelevated operating temperature T₂₃ which is higher than T₁₂. Thereceiver is similar to the receiver 700 in FIG. 7. The receiver 3132 hasan exposed opening of width 3142. The width 3142 is less than the widthof 3140 in order to improve overall efficiency of the receiver. Thefirst 3108 and second 3110 receiver sections are connected via a coupler(not shown) that connects the two receiver absorber tubes together. Thereceiver absorber tubes may be the same dimensions with differentexposed areas (e.g., different extent of insulation) or differentdimensions (e.g., smaller tube for 3110 as compared with 3108).

As shown in FIG. 20B, for a given mirror size and geometry, optimalreceiver performance at a given operating temperature occurs fordifferent receiver geometries. Receiver efficiency is a multiplicationof optical and thermal efficiency. Smaller exposed absorber surface areatends to result in higher thermal efficiency (lower thermal losses dueto smaller exposed area) but lower optical efficiency (high opticallosses due to smaller exposed area).

Additional receivers 3134, 3136 corresponding to additional sections3114, 3116 will typically operate at further elevated operatingtemperatures, each successively higher. The exposed width of eachsuccessive receiver 3134, 3136 may be less than the previous receivers(3132, 3134) in order to improve overall efficiency of the receiver.Each section may be connected via a coupler (not shown) that connectsthe two receiver absorber tubes together. The receiver absorber tubesmay be the same dimensions with different exposed areas (e.g., differentextent of insulation) or different dimensions (e.g., smaller tube).Overall solar field efficiency can be increased through this method.

Additionally, as shown in FIG. 32, receivers employing different shapesand technologies can be used in different receiver sections.

FIG. 32 displays another illustrative embodiment of the invention.Specifically, FIG. 32 shows solar power system 3200 which includes analternative embodiment of a solar field 3202 to replace or supplementsolar field 102 in FIG. 1 for a trough-based concentrating solar powersystem (100 in FIG. 1). The solar field is controlled to track the sunvia controller 3280.

The solar field 3202 comprises N rows (e.g., row 3204, row 3206),depicted in FIG. 32 as viewed from above, where each row comprises Mreceivers (e.g., receivers 3208 a, 3208 b; receivers 3210 a, 3210 b,receivers 3212 a, 3212 b; etc.) upon which solar radiation is focused byparabolic collectors (not depicted). The plurality of the N rows isindicated in FIG. 1 by horizontal ellipses (e.g., ellipsis 3216); theplurality of M receivers in each of the N rows is indicated in FIG. 1 byvertical ellipses (e.g., ellipses 3214 a, 3214 b). The number ofreceivers K in the solar field of the illustrative system 100 istherefore K=NM. In other embodiments, the number of receivers may varyfrom row to row.

In one state of operation, a heat-transfer fluid (e.g., molten salt,refractory oil, gas, pressurized water or steam) is admitted to an entrymanifold 3218 at a first temperature, T₁ (e.g., 200° C.). (In FIG. 32,any piping through which the heat-transfer fluid may flow in some stateof operation of the system 3200 is denoted by a thick dark line.) Theheat-transfer fluid is distributed by the entry manifold to the N rowsand, in each row, passes sequentially through the M receivers comprisedby that row. In general, an approximately equal amount of solarradiation Q_(c) is gathered by the collector associated with eachreceiver, and absorbed by the fluid in each receiver with an efficiencyη_(r). Thus, an approximately equal quantity of solar energyQ_(r)=Q_(c)×η_(r) is added to the heat-transfer fluid during its passagethrough each receiver. Each incremental set of the M receivers will thusraise the thermal energy and thus, for non-phase change heat transferfluids, raise the temperature of the heat-transfer fluid. The first setof receivers (3208 a and 3208 b), which consists of a given length ofreceivers (e.g., 40 meters), will raise the heat transfer fluid from T₁to a second elevated temperature T₂ (e.g., 250° C.) higher than the T₁.For simplicity, the average temperature T₁₂, (e.g., 225° C.) of thefirst set of receivers 3208 may be considered the average of T₁ and T₂(T₁₂=(T₁+T₂)/2). The second set of receivers (3210 a and 3210 b), whichconsists of a second given length of receivers (e.g., another 40 metersor some other length not necessarily equal to the first length), willraise the heat transfer fluid from T₂ to a third elevated temperature T₃(e.g., 300° C.). The second set of receivers 3210 operates at averagetemperature T₂₃ (e.g., 275° C.) higher than the average operatingtemperature, T₁₂, of the first set of receivers 3208. In the samemanner, the third set of receivers 3212 has a given length, inlettemperature T₃, elevated outlet temperature T₄ (e.g., 350° C.), andaverage operating temperature T₃₄ (e.g., 325° C.). Additional sets ofreceivers 3214 may raise the temperature further to the outlet operatingtemperature where it is admitted to an output manifold 3220 at a finaltemperature, T_(f) (e.g., 550° C.).

Detail 3230 shows an exemplary cross-section of the receiver in thefirst receiver sections 3208 a and 3208 b which typically operates atelevated operating temperature T₁₂. The receiver may be similar to thecommercially available vacuum tube receiver shown in FIG. 3. Thereceiver 3230 has a diameter of 3240 and an exposed circumference of pitimes the diameter 3240.

Detail 3232 shows an exemplary cross-section of the receiver in thesecond receiver sections 3210 a and 3210 b which typically operates atelevated operating temperature T₂₃ which is higher than T₁₂. Thereceiver may be a vacuum tube type receiver (not shown) as in 3230, butof different diameter, or may be, as shown, similar to the receiver 700in FIG. 7. The receiver 3232 has an exposed opening of width 3242. Theexposed width 3242 is less than the exposed circumference (pi times3240) in order to improve overall efficiency of the receiver. The first3208 and second 3210 receiver sections are connected via a coupler (notshown) that connects the two receiver absorber tubes together. Thereceiver absorber tubes may be the same dimensions with differentexposed areas (e.g., different extent of insulation) or differentdimensions (e.g., smaller tube for 3210 as compared with 3208). Invarious other embodiments, receivers may be used that comprise planartransparent aperture covers, lenticular aperture covers, partialaperture covers, selective insulated aperture covers, compound parabolicconcentration, and other aspects of the invention.

Additional receivers 3234, 3236 corresponding to additional sections3214, 3216 will typically operate at further elevated operatingtemperatures, each successively higher. The exposed width of eachsuccessive receiver 3234, 3236 may be less than the previous receivers(3232, 3234) in order to improve overall efficiency of the receiver.Each section may be connected via a coupler (not shown) that connectsthe two receiver absorber tubes together. The receiver absorber tubesmay be the same dimensions with different exposed areas (e.g., differentextent of insulation) or different dimensions (e.g., smaller tube).Overall solar field efficiency can be increased through this method.

FIG. 33 depicts an illustrative alternative assembly 3300 for thecollection of solar energy in the form of heat that may be part of alarger system for the conversion of solar energy to thermal and/orelectrical energy. Assembly 3300 may use less materials and be lowercost and/or higher performance than the standard assembly 200 displayedin FIG. 2. Assembly 3300 may be used in the solar field 102 of apower-generating system similar to that illustrated in FIG. 1 and mayuse receiver and mirror assemblies as described in FIGS. 4, 7, 11, 12,and other embodiments shown herein.

Illustrative assembly 3300 comprises a receiver tube 3302. Solarradiation (not shown, but similar to solar radiation depicted in FIG. 2)is reflected from a trough-shape collector 3306 having a paraboliccross-section and impinges on the receiver 3302. The receiver 3302 isheld in place at the focus of the parabolic trough 3306 by struts 3308.A supporting rod 3330 is also extended and supported by struts 3318 onthe reverse side of the mirror. The assembly comprising receiver 3302,rod 3330, struts 3308 3318, and trough 3306 is connected to a centralrod and joint mechanism 3314, which in turn is mounted upon supports3312, and can be rotated on a joint or bearing mechanism 3314 so thatthe trough 3306 faces the sun (i.e., is tilted at an angle equal to theelevation of the sun above the horizon at a given moment). Movement ofthe assembly 3300 for sun-track, or operation of other controllablefeatures of the assembly 3300, may be controlled by a controller (e.g.,computer) 3316.

Mirror 3306 is mounted on a thin structure which is not structurallystable without a truss-type support (210 as shown in FIG. 2) or otherstabilizing method. As illustrated in FIG. 33, a suspension cable (solidlines 3322, 3324, 3332, 3334) stabilizing structure can be used tostabilize and keep the mirror 3306 in its proper shape under the forcesof gravity, wind, and other forces. A lower suspension structure mayconsist of cables connected to the underside of the mirror 3306 and thinstructure at multiple points, including as illustrated cables 3322connected between the underside of the mirror structure to rod 3320,cables 3324 connected between the underside of the mirror structure torod 3320. This lower suspension arrangement including struts 3318maintains the shape of the mirror from flexing primarily in the upwarddirection as illustrated. Additional such cables (not shown) may extenddown the length of the mirror. A second upper suspension structure maycomprise cables connected to the topside of mirror 3306 and thinstructure at multiple points, including as illustrated cables 3332 aconnected between the underside of the mirror structure to receiver3302, cables 3334 a connected between the underside of the mirrorstructure to receiver 3302. This upper suspension arrangement includingstruts 3308 maintains the shape of the mirror from flexing primarily inthe downward direction as illustrated. Additional such cables, including3334 b and 3334 b, may extend down the length of the mirror.

The cavity receivers described in FIG. 4, FIG. 7, and other Figuresherein are ideal for connection to a suspension type structure, sincethey have a rigid non-glass top surface. For other receivers that do nothave a rigid or otherwise strong enough structure to support asuspension type cable connection, an additional rod (not shown) may beinstalled above or below the receiver.

Suspension-type support and stabilizing mechanisms are well known inbridge construction to use less material than truss-type support andstabilizing mechanisms in many applications. For mirror structures forsolar power towers, suspension-type support mechanisms developed bySolaflect (e.g., European Patent Application “Solar collector stabilizedby cables and a compression element,” EP 2215712 A1) show potential forreducing materials and cost, while maintaining or improving performance.

Various embodiments of the invention including a glass cover can employborosilicate crown glass, which typically has low dispersion, lowcoefficient of thermal expansion (CTE), and low refractive indices.Pyrex glass is such a material, with a specific example being Pyrex7740, manufactured by Corning Inc., having an optical transmittance of90-94% in the visible spectrum and a refractive index of 1.473 at 589 nm(peak solar). As is known in the art, the glass can be coated withanti-reflective (AR) coatings, such as silica coatings, which increasesthe optical transmission of its glass, for example from ˜92% to 96%.Such coatings are described in more detail the reference N. Benz, NextGeneration Receivers, NREL Trough Work Shop, March 7-8, Golden Colorado,2007, hereby incorporated by reference in its entirety.

The central absorber tube for various embodiments can be manufacturedfrom a variety of high temperature metals or ceramics with anillustrative example being stabilized austenitic steel, which has goodhigh temperature performance and is able to withstand repeated thermalcycling.

In various embodiments that eliminate vacuum, high temperature thermalinsulation is a required component. Various candidate materials existwith considerations including cost and low thermal conductivity, k, atboth intermediate and high temperatures. Some materials aresilica-based, including Microtherm MPS by Microtherm Group, a pyrogenicsilica, and Pyrogel XT by Aspen Aerogel, a silica aerogel. Theseinsulators have conductivity values on the order of 0.034 W/m·K at 800°C. and 0.089 W/m·K at 600° C. and are stable up to 1000° C. and 650° C.,respectively. As well, less expensive insulation (e.g., fiberglass,polyurethane) may be added outside the high temperature insulation or inother embodiments of the invention. Additional options for low and hightemperature insulations and their approximate performancecharacteristics are noted in Table 2.

TABLE 2 Examples of Thermal Insulation Options for Various EmbodimentsThermal Insulation Maximum Thermal Conductivity, k Stable Figure of at400° C. at 650° C. at 650° C. Density Temperature Price/ Merit, 400° C.Description Form Factor W/mK W/mK W/mK kg/m² ° C. m² 1000/(K

price/m³) Alumina silica ceramic fiber flexible or solid 0.09 128 >650$680 10.3 Calcium silicate solid 0.095 0.123 232 850 $2

10 5.0 Mineral 

flexible 0.12 0.175 0.22 126 850 $290 28.7 Milboard rock 

 + clay solid 0.11 946 >850 $8120 1.1 Fiberglass flexible or solid 0.074N/A N/A 48 up to 500 $080 20.5 Cellular Glass solid 0.13 N/A N/A 120 480$4650 1.7 Pedia

particulate 0.12 0.15  Microtherm 3.5* OD MPS solid 0.02 0.026 0.03 3201000 $6770 7.4 Pyrogel XT flexible 0.046 0.070 0.1  650 Pyrogel 6050flexible 0.03 650

indicates data missing or illegible when filed

Various embodiments may require reflective materials such as thecompound parabolic collector design described above. Specular surfacesmay be advantageous for such embodiments where as other embodiments maybenefit from diffuse reflection. A selection of example candidatematerials for reflectance applications are summarized in Table 3. Suchand other coatings and materials may also be added in variousembodiments to prevent radiation losses and reduce emissivity to nearzero.

For various embodiments, the absorber tube is covered with asolar-selective coating. The coatings can require high solar (for someembodiments the wavelengths are approximately λ≦3 mm) absorptivity, α,and low thermal (for some embodiments the wavelengths are approximatelyλ≧3 mm) emissivity, ε. In some embodiments of the invention employing avacuum, coatings otherwise susceptible to oxidation may be used.Stability at high temperature is a consideration for absorber coatings.Furthermore, the coefficient of thermal expansion of the coating andsubstrate must be considered with respect to thermal cycles.

Table 4 lists current and candidate absorber coatings and theirproperties. The ratio of solar absorptivity to thermal emissivity atvarious temperatures, when available, gives a common figure of merit tocompare solar-selective coatings. Accurate spectral andtemperature-dependent physical properties for solar-selective coatingsare difficult to obtain partly due to limitations of instruments capableof measurement at high temperatures.

TABLE 3 Examples of Reflective Material Options for Various EmbodimentsReflective Materials Coefficient Maximum Reflectance at of ThermalStable Typical Solar Weighted Thermal Temp. Expansion TemperatureThickness Price/ Material Hemispherical % Specular % Emissivity ε ° C.10⁻⁶/° C ° C. mils ft² Silver coating polished 93 90 0.01 40 720 0.0040.03 540 Silver metallized polymer 3M Solar Mirror Film 1100 94 95 70 654.6 $3 ReflecTech Mirror Film 93 94 55 60 4 $3 Aluminum highly polishedplate 88 85 0.038 200 22.2 600 32 $15 0.06 600 bright foil 85 0.04 20 6to 62 0.05 40 polished plate 80 0.095 100 Stainless Steel bright foil 630.05 30 17 870 25 $72 Chrome electro-plating polished 60 0.08 40 6.2 4270.5 $430 0.4 1100

However, even from single values for ε (commonly available up to 100°C.), we can extrapolate emissivity at higher Tabs to a firstapproximation for the next round of optical and thermal modeling. Costsare estimated in Table 3 relative to the baseline PTR70 coating, rangingfrom (−) slightly less expensive, to (++) substantially more expensive.Additional materials can be found in the reference by E. Kennedy and H.Price entitled “Progress in Development of High-TemperatureSolar-Selective Coating,” found in Solar Energy, vol. 2005, pp. 749-755,2005 and hereby incorporated by reference in its entirety.

FIG. 34A and FIG. 34B display two illustrations of selective solarabsorbers.

FIG. 34A displays an illustration 3402 of a conventional cermetselective solar absorbers, consisting of metal nanoparticles dispersedin a ceramic matrix, which have been developed for CSP. It is well knownin the art that metals have a high absorption coefficient in the solarspectrum, yet due to refractive-index mismatch with air a continuouslayer of metal reflects most incident light instead of absorbing it.Traditional cermet structures address this issue by incorporating gradedvolume fractions of metal nanoparticles (˜5-10 nm diameter) into ceramiclayers in order to tune the refractive-index profile of the coating formatching of optical impedance in the solar spectrum. A graded-indexcermet 3404 (also termed a graded cermet 3404) comprises multipleceramic layers, with increasing volume fractions of small metalnanoparticles from top to bottom. This graded cermet 3404 is depositedupon a substrate 3408 and an infrared reflector 3406 (“IR Reflector”).Design

TABLE 4 Materials table for Absorber Coatings Absorber Coatings Figureat of Solar Thermal Temp. Merit Relative Description Absorptivity αEmissivity ε ° C. α/ε Deposition Cost SOA materials Luz Black Chrome0.94 0.11 100 8.55 electro-deposition − 0.27 400 3.48 Luz Cermet 0.920.08 100 11.50 sputtering 0.15 400 6.13 Solel UVAC Cermet 0.955 0.076100 12.57 Sub-mm metalic dendrons in transparent matrix 0.14 400 6.82Siemens UVAC 2010 0.96 0.09 400 10.67 Siemens Al2O3 based cermet 0.960.1 400 9.60 + Siemens Mo—Al2O3 or W—Al2O3 0.96 0.16 350 6.00 planarmagnetron + ENEA-Italy HEMS08 coatings for moiten-sait HTFMo/Mo—SiO2(HMVF)/Mo—SiO2(LMVF)/SiO2 0.94 0.13 580 7.23 sputtering +graded W/W—Al2O3/Al2O3 0.93 0.1 400 9.30 sputtering + 0.14 580 6.64SCHOTT PTR70 absorber coating 0.95 0.1 400 9.50 unknown = SCHOTTmultilayer selective 7-layer coating 0.955 0.1 100 9.55 2-source PVDCandidate materials black chrome 0.916 0.109 100 8.40 − 0.22 400 4.160.239 450 3.53 0.257 500 3.58 semiconductor-pigmented paint Ge insilicone binder, on stainless steel substrate 0.91 0.7 100 1.30 painted− AMA multilayer coating, Al2O3—Mo—Al2O3 0.85 0.11 500 7.73 vacuumevaporation − Titanium Nitride TiNOx on Cu substrate 0.92 0.06 100 15.33evaporation (ARE) Double Mo—Al2O3 Cermet layer on Cu substrate 0.96 0.08350 12.00 vacuum co-evaporation + Mo—SiO2 double layer cermet 0.94 0.13580 7.23 sputtering + graded Pt—Al2O3 cermet on Pt coated fused quartzsubstrate 0.94 0.08 150 11.75 vacuum co-evaporation ++ graded Pt—Al2O3cermet on quartz + SiOx AR top layer 0.98 0.2 150 4.90 vacuumco-evaporation ++ ZrOxNy single layer/tandem thin film absorber 0.9 0.08327 11.25 sputtering Ti-based tandem absorber, TiAlN/TiAlON/Si3N4 0.930.15 82 6.20 Nanoparticles embedded in dielectric matrix 0.94 0.07 75013.43 = Sol-gel nano-structured absorber coating Low-e surface coatedwith a high solar-absorptance, 0.96 0.15 100 0.40 sol-gel ++nano-pinnacle structured selective layer, Ni2Sn3 alloy Nickel nanochainsin Al2O3 cermet 0.93 0.09 100 10.33 solution-chemicals-coating −optimization entails engineering the effective refractive index and filmthickness of each layer. In these structures, film thicknesses arecritical for optical performance, leading to stringent requirements onthickness control. Therefore, existing cermet fabrication techniques,such as sputtering, evaporation, and chemical vapor deposition,typically rely on vacuum. The relatively high cost and low throughput ofthese techniques limits cost reduction of CSP systems. Additionally,mid- to high-temperature selective solar absorbers that are stable inair remain a significant challenge because metal nanoparticles aretypically easily oxidized in air. Currently, most CSP systems utilizevacuum tubes to avoid high-temperature oxidation; vacuum failure is abottleneck for the lifetime of CSP systems.

FIG. 34B displays an illustration of a coating using a plasmonicnanochain cermet structure which can, in various embodiments, achievehigh solar absorptance and low thermal emittance at low fabricationcost. The plasmonic nanochain cermet structure 3422 can be fabricated bycost-effective solution chemical methods. As an example, a new Ninanochain-Al₂O₃ cermet is schematically shown in FIG. 34B. The Ninanochains 3424 consist of Ni nanoparticles ˜100 nm in diameter, anorder of magnitude larger than nanoparticles in traditional cermets. TheNi nanochains 3424 are dispersed in a ceramic matrix 3426 such asSiO_(x) upon a substrate 3428. This nanoparticle size is chosen suchthat absorption and scattering in the solar spectrum can besignificantly enhanced by optical excitation of surface plasmapolaritons in metal nanostructures. Experimentally this radius can bewell controlled by the molar ratio of Ni²⁺ to N₂H₄. Strong surfaceplasma polariton scattering from Au nanoparticles (50˜100 nm indiameter) towards Si has been utilized to increase the photocurrent inSi photodiodes and improve the energy conversion efficiency in thin-filmphotovoltaic devices. A challenge for solar absorber applications,however, is that the plasmonic resonances in noble metals (like Au, Ag)are too narrow to cover the entire solar spectrum. For broadbandplasmonic absorbers, Ag crossed gratings have been designed andfabricated by e-beam lithography. In contrast to noble metals,ferromagnetic metals such as Ni and Fe have a higher damping coefficientso that the surface plasma polariton resonance appears broader inspectrum. This feature is advantageous for solar thermal absorbers.

FIG. 35 displays an illustrative plot for extinction, absorption, andscattering for various length nanochains using a plasmonic nanochaincermet structure which can, in various embodiments, achieve high solarabsorptance and low thermal emittance at low fabrication cost. Asillustrated in FIG. 35, the optical response (wavelength dependentextinction, absorption, and scattering) can be optimized by tailoringthe length of Ni nanochains. A plot of efficiency factor versus opticalwavelength is displayed for a single nanosphere 3502, a 2-nanospherechain 3504, a 6-nanosphere chain35, and a 10-nanosphere chain 3508.Nanosphere diameter is 80 nm. Efficiency factors presented here areabsorption/scattering/extinction cross-sections normalized by geometricarea of the nanostructures. Incident light is polarized along nanochain.For incident light polarized along the nanochain (x-polarization), theoptical response spectrum can be extended from λ˜1000 nm to λ˜2500 nm byvarying the length of the nanochain, covering >99% of optical energy inthe solar spectrum. The optical field is found to be strongly enhancedat the narrow gaps between nanospheres due to near-field plasmoniceffect, an advantage over nanorod structures in terms of surface plasmapolariton enhancement. A 6-nanosphere chain 3506 shows a stronger fieldenhancement than the 2-nanosphere one 3504 at λ=2500 nm, confirming thatthe increase in absorption at longer wavelengths (λ>2000 nm) is indeedrelated to the surface plasma polariton effect. On the other hand, the˜100 nm diameter Ni nanoparticles are small enough that long-wavelengthMIR photons from thermal radiation cannot resolve them. Consequently,these MIR photons see the Ni nanochain network as a continuous metalsheet and are reflected back to the CSP system, minimizing thermalemittance losses. Optical performance is inherently determined by thestructure of plasmonic Ni nanochains instead of layer thicknesses,greatly facilitating low-cost, solution-chemical processing.

Antioxidation behavior may be achieved at >500° C. by the use of aSi-rich ceramic matrix such as SiO_(x) (x=1.5-1.9) for two reasons: (1)it is possible to form metal-silicon bonds similar to those in silicidematerials that shows much better thermal stability than metals. (2) theSiO_(x) matrix provides a high refractive index contrast to the Ninanoparticles, which enhances the plasmonic scattering effect for betteroptical performance. Antioxidation behavior at 500 C has beendemonstrated in air.

FIG. 36 shows scanning electron microscopy images of example Ninanochains as synthesized 3602 and in a coating JJ04. The Ni nanochainsare coated and annealed as a Ni—Al₂O₃ composite at an annealingtemperature of 400° C. in N₂. In their preliminary studies, the JifengLiu Group at Dartmouth has demonstrated spectrally selective plasmonicNi nanochain cermet structures created using solution-chemicalfabrication. The Ni nanochains form a 3D network as shown in FIG. 36,which offers stronger overall solar absorption due to multiplescatterings. Since the nanochains are randomly oriented, the spectralresponse becomes polarization independent.

FIG. 37 displays an illustrative plot for reflectance for a plasmonicnanochain cermet structure which can, in various embodiments, achievehigh solar absorptance and low thermal emittance at low fabricationcost. FIG. 37 shows the reflectance as a function of wavelength for aplasmonic nanochain cermet structure 3702. Also plotted are thenormalized AM 1.5 solar spectrum 3704 and normalized 400 C blackbodyradiation spectrum. Based on the reflectance spectrum 3702, the Ninanochain-Al₂O₃ cermet structure spin-coated on stainless steelsubstrate demonstrates a high overall solar absorptance of 93% and anoverall thermal emittance of 9%. This optical performance is comparableto vacuum deposited multilayer cermets while the fabrication process ismuch less expensive. The performance can be further optimized by finetuning the nanoparticle sizes to better match the solar spectrum and/orusing lower refractive index material such as SiO₂ instead of Al₂O₃.

FIG. 38A shows the molecular structure of hydrogen silsesquioxane; FIG.38B shows a plot showing x-ray diffraction peak intensity ratio data.The Liu Group has also studied the passivation of metal nanochains usingnew ceramic matrix materials to prevent oxidation of thesenanostructures at high temperatures (>500° C.) in air. Preliminarystudies tested hydrogen silsesquioxane (HSQ). The molecular structure3802 of HSQ is shown FIG. 38A. HSQ has also been applied as a molecularprecursor to form Si—SiO₂ nanocomposite at an annealing temperatureof >1000° C. to achieve interesting light-emission properties. A Si-richmatrix could lead to the formation of metal-silicon bonds similar tothose in silicide materials that shows much better thermal stabilitythan metals. The Ni nanochains were ultrasonically dispersed in to HSQsolutions, spin-coated on stainless steel substrates, and annealed at700° C. in N₂ to form the cermet coating. In sharp contrast to Ninanochain in Al₂O₃ matrix, which significantly oxidizes after annealingat 450° C. for 15 min in air, X-ray diffraction analysis show that theNi nanochains in HSQ matrix had little oxidation even after annealing at450° C. for 2 h in air. As shown in FIG. 38B, the ratio of NiO(200) toNi(111) peak intensity in X-ray diffraction analysis is an order ofmagnitude lower in HSQ (curve 3812, circular markers) compared to Al₂O₃matrix (curve 3814, square markers). The Ni nanochain-HSQ cermet coatingwas further annealed at 500° C. for an additional hour, and this ratioremained the same, indicating no further oxidation even at 500° C. Thisis a significant improvement compared to Ni nanochains embedded in Al₂O₃matrix, and offers the promise that oxidation resistance at >550° C. canbe achieved with process optimization. In an embodiment of theinvention, oxidation resistance may be further improved through theinsertion (or flow) of an inert gas through the aperture into the cavityto improve environmental conditions (i.e. displace water).

Embodiments of the invention that use such a solar selective coatingthat does not degrade or oxidize during high temperature operationwithin allows substantial improvements in performance and cost oftrough-based concentrating solar plants.

It is expected that Norwich Technologies Inc. receiver will be not onlymore reliable and higher performing than existing receivers, but alsoless expensive. The objective of lower cost is realized by the NorwichTechnologies designs shown herein (e.g., FIG. 4, FIG. 7, FIG. 11) asthey eliminates the vacuum requirement, removes the thick outer glassshell, and employs simpler coatings. The elimination of the vacuumreduces not only manufacturing costs (e.g., vacuum, bellows, andgetters) but also maintenance and operation costs (e.g., identifying andreplacing vacuum failures, avoiding most cases of glass breakage, androutine getter maintenance).

FIG. 39 illustrates in graphical form the results of an analysisrelative to the SOA. The estimated cost breakdowns are broken intocategories and prices normalized to the total cost of the SOA SchottPTR70 receiver. Cost breakdown categories include absorber tube 3902,absorber coating 3904, vacuum accessories 3906, reflective coating 3908,glass envelope 3910, anti-reflective coating 3912, and insulation 3914.NT designs are shown based on designs shown in early figures includingFIG. numbers (FIG. 4, FIG. 7, FIG. 11).

DEFINITIONS

As used herein, the term “light” includes but is not restricted to thevisible portion of the electromagnetic spectrum.

As used herein, the terms “pipe,” “piping” and the like refer to one ormore conduits that are rated to carry gas or liquid between two points.Thus, the singular term should be taken to include a plurality ofparallel conduits where appropriate.

Recording the results from an operation or data acquisition, such as forexample, recording results at a particular frequency or wavelength, isunderstood to mean and is defined herein as writing output data in anon-transitory manner to a storage element, to a machine-readablestorage medium, or to a storage device. Non-transitory machine-readablestorage media that can be used in the invention include electronic,magnetic and/or optical storage media, such as magnetic floppy disks andhard disks; a DVD drive, a CD drive that in some embodiments can employDVD disks, any of CD-ROM disks (i.e., read-only optical storage disks),CD-R disks (i.e., write-once, read-many optical storage disks), andCD-RW disks (i.e., rewriteable optical storage disks); and electronicstorage media, such as RAM, ROM, EPROM, Compact Flash cards, PCMCIAcards, or alternatively SD or SDIO memory; and the electronic components(e.g., floppy disk drive, DVD drive, CD/CD-R/CD-RW drive, or CompactFlash/PCMCIA/SD adapter) that accommodate and read from and/or write tothe storage media. Unless otherwise explicitly recited, any referenceherein to “record” or “recording” is understood to refer to anon-transitory record or a non-transitory recording.

As is known to those of skill in the machine-readable storage mediaarts, new media and formats for data storage are continually beingdevised, and any convenient, commercially available storage medium andcorresponding read/write device that may become available in the futureis likely to be appropriate for use, especially if it provides any of agreater storage capacity, a higher access speed, a smaller size, and alower cost per bit of stored information. Well known oldermachine-readable media are also available for use under certainconditions, such as punched paper tape or cards, magnetic recording ontape or wire, optical or magnetic reading of printed characters (e.g.,OCR and magnetically encoded symbols) and machine-readable symbols suchas one and two dimensional bar codes. Recording image data for later use(e.g., writing an image to memory or to digital memory) can be performedto enable the use of the recorded information as output, as data fordisplay to a user, or as data to be made available for later use. Suchdigital memory elements or chips can be standalone memory devices, orcan be incorporated within a device of interest. “Writing output data”or “writing an image to memory” is defined herein as including writingtransformed data to registers within a microcomputer.

“Microcomputer” is defined herein as synonymous with microprocessor,microcontroller, and digital signal processor (“DSP”). It is understoodthat memory used by the microcomputer, including for exampleinstructions for data processing coded as “firmware” can reside inmemory physically inside of a microcomputer chip or in memory externalto the microcomputer or in a combination of internal and externalmemory. Similarly, analog signals can be digitized by a standaloneanalog to digital converter (“ADC”) or one or more ADCs or multiplexedADC channels can reside within a microcomputer package. It is alsounderstood that field programmable array (“FPGA”) chips or applicationspecific integrated circuits (“ASIC”) chips can perform microcomputerfunctions, either in hardware logic, software emulation of amicrocomputer, or by a combination of the two. Apparatus having any ofthe inventive features described herein can operate entirely on onemicrocomputer or can include more than one microcomputer.

General purpose programmable computers useful for controllinginstrumentation, recording signals and analyzing signals or dataaccording to the present description can be any of a personal computer(PC), a microprocessor based computer, a portable computer, or othertype of processing device. The general purpose programmable computertypically comprises a central processing unit, a storage or memory unitthat can record and read information and programs using machine-readablestorage media, a communication terminal such as a wired communicationdevice or a wireless communication device, an output device such as adisplay terminal, and an input device such as a keyboard. The displayterminal can be a touch screen display, in which case it can function asboth a display device and an input device. Different and/or additionalinput devices can be present such as a pointing device, such as a mouseor a joystick, and different or additional output devices can be presentsuch as an enunciator, for example a speaker, a second display, or aprinter. The computer can run any one of a variety of operating systems,such as for example, any one of several versions of Windows, or ofMacOS, or of UNIX, or of Linux. Computational results obtained in theoperation of the general purpose computer can be stored for later use,and/or can be displayed to a user. At the very least, eachmicroprocessor-based general purpose computer has registers that storethe results of each computational step within the microprocessor, whichresults are then commonly stored in cache memory for later use, so thatthe result can be displayed, recorded to a non-volatile memory, or usedin further data processing or analysis.

THEORETICAL DISCUSSION

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. A system for generating energy from solarradiation as part of a solar power system, said system comprising: aplurality of linear receivers, each of said plurality of linearreceivers including at least a solar radiation absorbing elementdesigned to absorb an incident flux of solar radiation and transfer anabsorbed flux of energy to a heat transfer medium, said heat transfermedium designed to receive and transport at least a portion of saidabsorbed flux of energy, at least a portion of said radiation absorbingelement being covered with a solar selective absorber, said solarselective absorber having a thermal emittance value and an opticalabsorptance value, said optical absorptance value being different fromsaid thermal emittance value; a parabolic trough mirror collector forconcentrating solar radiation onto said plurality of linear receivers; acontrol system for directing said parabolic trough mirror at the sun,wherein said heat transfer medium circulating in a first receiver insaid plurality of linear receivers is heated by solar radiation from afirst elevated temperature T1 to a second elevated temperature T2 over afirst distance corresponding to a length of said first receiver and saidheat transfer medium circulating in a second receiver in said pluralityof linear receivers is heated by solar radiation from a third elevatedtemperature T3 to a fourth elevated temperature T4 over a seconddistance corresponding to a length of said second receiver, whereT4>T3≧T2>T1, said first receiver and said second receiver havingstructures designed for operation in different temperature ranges. 2.The linear solar receiver of claim 1, wherein the portion of theradiation absorbing element being covered with the solar selectiveabsorber for said first receiver is greater than the portion of theradiation absorbing element being covered with the solar selectiveabsorber for said second receiver.
 3. The linear solar receiver of claim1, wherein said first portion of said outer surface of said solarradiation absorbing element is substantially planar.
 4. The linear solarreceiver of claim 1, wherein said first portion of said outer surface ofsaid solar radiation absorbing element comprises a fraction in the rangeof 0.50 to 0.20 of an area of said outer surface of said solar radiationabsorbing element determined on a per unit length basis.
 5. The linearsolar receiver of claim 1 further comprising a glass cover enclosing thesolar radiation admitting region.
 6. The linear solar receiver of claim5 wherein an inert gas is introduced into the radiation admittingregion.
 7. The linear solar receiver of claim 1, wherein said interiorsurface of said solar radiation admitting region forms a compoundparabolic collector.
 8. The linear solar receiver of claim 1, whereinsaid interior surface of said solar radiation admitting region is areflective surface.
 9. The linear solar receiver of claim 1, whereinsaid heat transfer medium is selected from the group consisting of amolten solar salt, a molecular silicone based fluid, and steam.
 10. Thelinear solar receiver of claim 1, wherein said linear receiver has athermal efficiency, said thermal efficiency selected from the groupconsisting of 94 percent at 450 degrees Celsius and 92 percent at 500degrees Celsius, 89 percent at 550 degrees Celsius, 85 percent at 600degrees Celsius, 80 percent at 650 degrees Celsius.
 11. The linear solarreceiver of claim 1 in combination with an energy collection systemconfigured to operate a Carnot cycle energy recovery machine.
 12. Thelinear solar receiver of claim 1 in combination with a plurality linearsolar receivers, said linear solar receivers each including at leastsolar radiation absorbing elements, adjacent solar radiation absorbingelements forming a nearly continuous absorbing surface.
 13. The linearsolar receiver of claim 1 in combination with a plurality linear solarreceivers, a first one of said plurality of receivers operating at afirst temperature and a second one of said plurality of receiversoperating at a second temperature, said first receiver and said secondof said plurality of receivers having different designs, said first andsaid second temperatures being different.
 14. The linear solar receiverof claim 1, further comprising a symmetric parabolic trough collectormirror structure having a rim angle of less than 75 degrees, saidsymmetric parabolic trough collector mirror structure focusing saidincident flux of solar radiation on said solar radiation absorbingelement, said linear solar receiver disposed between said symmetricparabolic trough collector mirror structure and the sun.
 15. The linearsolar receiver of claim 1, further comprising a symmetric parabolictrough collector mirror structure, said symmetric parabolic troughcollector mirror structure being held in a substantially rigid form withcable suspension.
 16. The linear solar receiver of claim 1, wherein atleast a portion of said absorbed flux of absorbed energy is used toperform an action chosen from the group consisting of generatingelectricity and completing an industrial process.
 17. The linear solarreceiver of claim 1, wherein said solar selective absorber is aplasmonic nanochain cermet structure.